There is an interesting transposition from the Schliemann into a King's Gambit declined (or from the From Gambit Declined). I noticed that some month ago but today I look at it again with interest.
Here is the line beginning from the White side:
1.e4 e5 2.f4 Bc5 3.Nf3 d6 4.Nc3 Nf6 5.fxe5!? dxe5 (or 1.f4 e5 2.fe5 d6 3.e4 de5 4.Nf3 Bc5 5.Nc3)
From the Black side:
1.e4 e5 2.Nf3 Nc6 3.Bb5 f5 4.d3 fxe4 5.dxe4 Nf6 6.Bc4!?Where we reach exactly the same position but only the color change.
What is interesting from the King's Gambit version: It's White who does the first marginal move: 5.fxe5 (only 17 games at this point according to chesslive!). So we could imagine that the position is now good for Black, at least equal. Thomas Johansson (in his website) did an article about that marginal line for White where there is a dangerous try for Black after 5..dxe5 6.Bc4 Nc6 7.d3 Ng4!? which is probably still unclear.
Now let's see the same position via the Schliemann: White's only "marginal move" is 6.Bc4 where we can say the same comment: "White is still at least equal or even better, where there is still an interesting attack with Ng5", but it has been only played about 4 times according to chesslive.
So what, Black is ready to play in thousand of King's gambit against a marginal move as 5.fxe5 but when they get White, they don't want to force that possible position with 6.Bc4? Why? That's probably for a psychological reason, as Bb5-Bc4 seems to be a waste of tempo, but when I see the other try from White side (Anand-Carlsen or 2700's-Radjabov) that's not necessary promising. So I imagine that people are not really aware of 6.Bc4 being a transpotition into a King's gambit, or they think it's equal, and that's ok with Black but not with White. That's remind me of 2 articles in chesscafe: "switching color"
http://www.chesscafe.com/text/kaiss30.pdfAnother amazing curiosity, when I did my position research after the Schliemann's line 1.e4 e5 2.Nf3 Nc6 3.Bb5 f5 4.d3 fxe4 5.dxe4 Nf6 6.Bc4, I noticed that it was still White who has to play here, so there are about 40 games where Black's player reached that position a tempo down! Usually like this: 1.d4 e5 2.dxe5 Nc6 3.Nf3 f6?! 4.e4 fxe5 5.Bc4 Nf6 where we get the position and White has 6.Ng4 where after 6..Bc5 7.Nxf7, the game is like a Traxler counter-gambit Without the white's d-pawn!
A final conclusion might be for the King's Gambit player: watch all these Schliemann's player who is ready to play your position (even some crazy guys like Stummer wanted to play that a tempo down) after the marginal 5.fxe5!? when you still play like in the books.