Back in the series starting with "Stats (1)" – where I took a look with ChessBase's "Opening Report" at the games in my Jerome Gambit (1.e4 e5 2.Nf3 Nc6 3.Bc4 Bc5 4.Bxf7+) database – I pointed out that
"the 'Critical line' for White [after 4...Kxf7] in which he scored only 29%, is 5.Nxe5+ Kf8 6.Nxc6 dxc6, which certainly bears some looking into."
HISTORY
As early as his first Jerome Gambit article (Dubuque Chess Journal April, 1874), Alonzo Jerome considered the possibility that Black might refuse to capture the second piece with 5...Nxe5, and instead play for King safety instead with 5...Kf8.
In a letter to the American Chess Journal, March 1877, Jerome wrote
5...Kf8 leaves White's pawns intact while Black has lost two strong pawns and doubled another. This defense was adopted by G.J. Dougherty of Mineola, NY, a strong amateur, against whom I first played the opening, and I think he will agree that 5...Kf8 is not a good defense. He generally played 6.bc and that was the play of Mr. J. C. Young of Danville, KY, who subsequently abandoned the game. Why, I do not know, as it was not necessarily lost to either of us. It is a question with which Pawn it is best to take.
The earliest game example that I have of this line is Jerome - Brownson, Iowa, USA, 1875 (1/2-1/2, 29) [correcting a misstatement in "Jerome Gambit and Vlad Tepes..."] – Brownson being the editor of the Dubuque Chess Journal – the game appearing in the March 1875 issue.
The Mexican chess champion, Andres Clemente Vazquez, familiar with the DCJ, played the Jerome Gambit in his second match against Carrington in 1876, and met 5...Kf8, winning in 43 moves.
This was, in fact, the defense that Hallock, the skeptical editor of the American Chess Journal, used in a correspondence game played “by special request” to test the gambit (ACJ February,1877) – defeating the promising young player, D.P. Norton, in 18 moves.
The 5...Kf8 defense received a great boost from an article in the "Chess for Beginners" series by Lt. Sorensen in the Nordisk Skaktidende of May 1877 – which was translated into several languages and printed around the world. Sorensen recommended 5...Kf8 as “more solid and easier to manage.”
After 6.Nxc6 dc (Jerome gave 6…bc 7.d4 “putting Black’s KB out of play”) analysis has generally followed Jerome – Brownson, 1875, with 7.O-O Nf6 8.Qf3 (Sorensen said 8.e5 would be met by 8…Bg4 9.Qe1 Kf7! which was how Norton – Hallock had continued ) Qd4 9.d3 Bg4 10.Qg3
At this point, Brownson played 10…Bb6. Jerome responded with 11.e5, and drew the game, with help from his opponent, in 29 moves. Brownson (DCJ, March, 1875) suggested 11.Kh1 and 12.f4 as an improvement for White.
Sorensen (NS, May, 1877) gave the alternative line 10…Bd6, attacking White’s Queen, and followed this up with 11.Bf4 g5 12.Bxd6+ cd 13.h3 Be6 14.Qxg5 Rg8 15.Qh6+ Ke7 16.Nc3 Rg6 17.Qh4 Rag8 with a better game for Black.
However, S.A. Charles – see "A Jerome Discovery (Afterword)" – in the Pittsburgh Telegraph a few years later (April 27, 1881) offered 11.c3 as an improvement, suggested to him by Jerome, which they both believed reversed the valuation of the line.As an historical aside, later sources, relying on Sorensen’s analysis, miss 11.c3; those that follow Charles’ work, based on his Brentano's Chess Monthly article (October 1881) or on the American Supplement (1884) – see "A Jerome Discovery (Afterword)" – include it.
There has been little change in the assessment of this line during the ensuing 125 years.
7.f4 Qf6 
11...Bd4 is not a "bad" alternative, but iltimately failed against White's active play in blackburne - drewbear, chessworld, 2008: 12.Rf1 Ke6 13.Nb5 Bb6 14.Bf4 d6 15.0-0-0 a6 16.Nc3 Bd7 17.Rf3 Rhf8 18.Rdf1 Ng4 19.h3 g5 20.Bxg5 Rxf3 21.Rxf3 Ne5 22.Rf6+ Ke7 23.Nd5+ Ke8 24.Rh6 Kf7 25.Nf6 Be8 26.Nxh7 Kg7 27.Nf6 Bg6 28.Nd5 Rf8 29.Ne7 Nf7 30.Rxg6+ Kh7 31.Bf6 Ne5 32.Rg7+ Kh6 33.Nf5+ Kh5 34.g4+ Black resigned
4...Kxf7 5.Nxe5+ Nxe5 6.Qh5+ Ng6
Improving on 6...g6 7.Qxe5 Nf6 from 
