[15+10. Weiß nimmt einen Zug zurück; dann Matt in einem Zug!]
Retract -1. O-O-O !!. Now it can be proven that Bl.
last move is -1 ... g7-g5:
"After" the retraction of -1. O-O-O, single bl. cap is h6xBg5. Wh.
caps include b2xc3, c2xd3, exf and Bc8 at home. bPe7 could not promote
on e1 because Wh. may still castle. So that bPe7 was captured on its
column. Same for bPa7, so that all caps are localized. bPc7 did promote
on c1=B. We may also precise that, because wKe1 did never move, the
bRf2 reached f2 through f3 so that exf really was e2xf3.
The position unlocks by reconducting wBh7 on f1 and retracting e2xf3:
-1 ... g7-g5 -2. Be4-h7 g5-g4 -3. Bc6xPe4 e5-e4 -4. Ba4-c6 e6-e5 -5.
Bd1xPa4 a5-a4 -6. Be2-d1 a6-a5 -7. Bf1-e2 a7-a6 -8. e2xQ/Nf3 and the
position unlocks just before Bl. is retropat. Retracting -1 ... g6-g5??
-2. Bg8-h7 g7-g6 -3. Bh7-g8 is too slow.
-1 ... g7-g5 is the only possible last move (indeed,
last 13 single moves are completely determined). So that forward
1. h5xg6 ep # works.
Solution by Philippe Schnoebelen