Surprisingly, the author of the History of Punch (1895) was ignorant of its first appearance in that magazine. He wrongly ascribed a misquoted version of the quip to the man who edited the magazine in 1857.
What seems to be the first correct attribution in print appeared in the endlessly diverting journal Notes and Queries. In 1853 two of its frequent contributors wrote — in the pleasant and somewhat quaint style of that journal — to supply information lacking to another. As you see, the first is correct, the second mistaken.
In 1881 a person going by the name of Bubbles also pointed out that Mr. Punch was responsible for the pun, but, like the author of the History of Punch, Bubbles gave an inaccurate version of the quip. This item appeared in the "Notices to Correspondents" section of Monthly Packet of Evening Readings for Members of the English Church
In 1889 the New York Times correctly cited "Napier's famous dispatch" as a joke, but mistakenly says it was committed not by Mr. Punch but by General Napier himself.
In 1904 a frequent contributor to the correspondents' columns of the Times' Book Review, wrote to say "It was Punch's idea that Sir Charles Napier.., the conqueror of Scinde, should so have announced his conquest."
In 1907 a correspondent to a journal devoted to the study of missions credited not Mr. Punch but an actual person with the pun.
Although this attribution has the ring of truth, it's odd that either Mrs. Macintosh or the editor got the punster's name wrong. It was not Catherine Wentworth but Catherine Winkworth, a translator and leader in the struggle for women's rights. Before the year was out the lexicographer, A.L. Mayhew, gave Notes and Queries readers the information that appeared in The East and the West and said he saw nothing to contradict the claim.
In 1913, Walter Woollcott (father of the famous Alexander) wrote a long piece on the peccavi joke in the "News for Bibliophiles" column of the Nation. In it he listed some of those who got their facts wrong. He also listed people who had been named as author and found no credible evidence for any but Catherine Winkworth.
The most interesting and extended investigation of the pun appears in the Presidential Address which the Indologist, Wendy Doniger, gave to the Annual Meeting of the Association for Asian Studies in March 1999. She quoted the joke as it appeared in Punch, gave full explanation and context, and listed a few who got the attribution wrong (the authors of a 1990 biography of Napier and of an article in the online Encyclopedia Britannica as well as paleontologist and historian, Stephen Jay Gould). She also gave her opinion that an editor of Punch was the inventor. However, she did not refer to, and apparently did not know of Mrs. Macintosh's letter to The East and the West or A.L. Mayhew's reference to it in Notes and Queries. Had she seen it, I believe she might have agreed that evidence supports Catherine Winkworth as the originating wit.
I believe also that she would have been pleased that Winkworth was the most-likely source since her thesis was that we should not be too quick to condemn the Victorians for their writings about India, but rather recognize the complex response of Britons to their imperial expansion in the sub-continent, and it's an indicator of this complexity that Winkworth was no typical Victorian John Bull. She was known in her time not just as a translator of German hymns but also as a fierce proponent of university education for women and women's suffrage.
As I say, I think Wendy Doniger would have appreciated learning about this unexpected source of the joke and appreciated, as well, the irony of its source in an untypical Victorian, since her main thesis concerns a natural tendency among people of our generation to condemn the 19th-century's dead white males, who — as "Dead White Male Orientalists" — wrote about India. By Orientalists, she meant the academic, cultural, and literary authors who wrote about the Indian sub-continent. She quoted the cultural critic, Lee Siegel, on the contributions that these men made: "Those hegemonic, imperialist, Euro-centric colonialists were such amazing writers and they knew so much more about India than all of us. They could ride horses, too."
Having begun with the Napier-pun and its interplay of Scinde and sin, she developed her main theme as an extended riff on the complex aspects of the Horse as cultural icon and "contradictory symbol of human political power." She said "horses offer a paradigm for us to use in our struggle to come to terms with the blotted copybook bequeathed to us by British Orientalism" and, having led her hearers (and us readers) through an extensive exploration of Indic horsiness her final thoughts were these: "The Hindu villagers' ability to appreciate the beauty and power of a creature that was the instrument of great political injury to them might inspire us to try once again to appreciate the tarnished but precious gift bequeathed to us by British Orientalists. Sometimes we cannot help looking a gift horse in the mouthpiece, or even in the ideology, but we can still accept the gift. Flawed as they are, the Orientalists are our ancestors, and, as Hamlet wisely cautioned, 'Use each man after his own deserts, and who shall 'scape whipping?'"
Monthly packet of Evening Readings for Members of the English Church, 3rd Series, Vol 2, edited by Charlotte Mary Yonge, Christabel Rose Coleridge, and Arthur Innes (J. and C. Mozley, 1881)
Musing about Peccavi and Twitter and accessibility
The American historical reviewVolume 9 (American Historical Association, 1904) Extract: 'An occasional good story is told in the volumes, as one of Soyer, the great French chef, who put on his irascible wife's tombstone "Soyez tranquille". India developed some bon mots, as after the relief of beleaguered Lucknow, one of Clyde's officers telegraphed home "Nunc fortunatus sum", i.e., "I am in luck now." Was it Napier in 1843 who sent the despatch "Peccavi", i.e., "I have Sindh"?' (From a review of Wolseley, The Story of a Soldier's Life by Theodore Avrault Dodge).
Last word by Ben Macintyre, Times (of London), March 4, 2006
Notes and queries, Volume 116, edited by William White (London, John C. Francis, 1907)
The East and the West, Volume 5 (Society for the Propagation of the Gospel in Foreign Parts, Great Britain, 1907)
Memorials of two sisters:
Susanna and Catherine Winkworth, edited by Margaret J. Shaen (Longmans, Green, 1908)
Catherine and Susanna Winkworth From: Dictionary of National Biography
Charles James Napier in wikipedia
Rambles in books edited by Charles Francis Blackburn (S. Low, Marston & Company, 1893)
The history of "Punch" by Marion Harry Spielmann, Volume 1 (Cassell and company, limited, 1895)
ON LANGUAGE; EXIT FOR HAIGSPEAK by William Safire, The New York Times, July 11, 1982
Democritus in London, with the mad pranks and comical conceits of Motley and Robin Good-fellow, to which are added notes festivous, etc. by George Daniel (W. Pickering, 1852)
"INDIA AND AFGHANISTAN (1815-1869) by William Lee-Warner in "The Cambridge modern history, Vol 11, Baron Acton, Ernest Alfred Benians (Cambridge University Press, 1909)
"Notes for Bibliophiles" by Walter Woollcott in The Nation Vol 96 (The Nation Company, 1913)
A history of modern England, Vol 1, by Herbert Woodfield Paul (The Macmillan company, 1904)
"Peccavi", Letter to the Editor of the New York Times Book Review by E.A. Hart, October 15, 1904
American notes and queries Vol 4, edited by William Shepard Walsh, and others (W.S. and H.C. Walsh, 1890)
"'I Have Scinde': Flogging a Dead (White Male Orientalist) Horse" and it appears in The Journal of Asian Studies (Vol. 58, No. 4, Nov., 1999 — JSTOR URL: http://www.jstor.org/stable/2658491)
 Handy-book of literary curiosities by William Shephard Walsh (London, Gibbings, 1894)
 The reference to the MP appears in Monthly packet of Evening Readings for Members of the English Church, 3rd Series, Vol 2, edited by Charlotte Mary Yonge, Christabel Rose Coleridge, and Arthur Innes (J. and C. Mozley, 1881). The compiler of puns was none other than William Walsh: American notes and queries Vol 4, edited by William Shepard Walsh, and others (W.S. and H.C. Walsh, 1890). The two historians were William Lee-Warner and Herbert Woodfield Paul ("INDIA AND AFGHANISTAN (1815-1869) by William Lee-Warner in "The Cambridge modern history, Vol 11, Baron Acton, Ernest Alfred Benians (Cambridge University Press, 1909) and A history of modern England, Vol 1, by Herbert Woodfield Paul (The Macmillan company, 1904) respectively). Safire's three mistakes appeared in his column "On Language" in the New York Times on July 11, 1982, August 30, 1987, and November 21, 1993.
 The history of "Punch" by Marion Harry Spielmann, Volume 1 (Cassell and company, limited, 1895)
 Notes and queries, Volume 116, edited by William White (London, John C. Francis, 1907)
 The address is entitled "'I Have Scinde': Flogging a Dead (White Male Orientalist) Horse" and it appears in The Journal of Asian Studies (Vol. 58, No. 4, Nov., 1999 — JSTOR URL: http://www.jstor.org/stable/2658491).