Shortest-proof-game challenge

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@6861

I am not sure this is legal.
Black's last move must have been ...Rxf1+ and only 1 white piece is missing, the queen, so black's last move must have been ...RxQf1+. 
Black has 2 light square bishops and 3 rooks, so 2 black pawns must have underpromoted to a rook and a bishop. The underpromotion to a bishop could only have happened on either d1 or f1. A black pawn could not reach d1 without a capture, thus black must have promoted ...f1=B. Thus the underpromotion to rook must have happened on h1: ...h1=R.
White has 3 doubled pawns, thus must have captured 4 black men: 2 knights and 2 pawns that did not underpromote.

This was a pretty difficult position. I knew there was some kind of sliding puzzle with the pieces in the bottom ranks but it wasn’t easy to resolve it all. Eventually I managed to make a 113-move proof game which reaches the position, but not without breaking the 50-move rule.

Here's how to avoid the 50-move rule. Same retraction sequence to get the Black Bishop to g1, then a quick un-promotion of Bf1 to reset the counter.

This is really a Retro, not a Proof Game, so I stopped retracting after unpromoting bQ on a1. The rest is easy.

Ah, I completely overlooked that possibility… but hang on. Isn’t it impossible to reach this arrangement of pieces?

White has exhausted his captures with pawns, so Nxg3 was not played last. hxg5 can’t be retracted since h1 was always occupied by the king so a black piece cannot unpromote there yet. Retracting c2-c3 is useless. d2-d3 would cause problems with wBh2 getting back home, since it is original. And retracting exd3 requires the uncapture of the black d-pawn, but then the d-pawns can’t get around each other. Because of this, wPd4 is stuck and bK can’t retract to e5. Everything else is jammed in.

It seems that avoiding the 50-move rule is not that simple…

Yep, shoulda stayed hidden behind Samuel...

@6867

First try:

@6870

Shorter:

@6872

Reply. Then click on the top left 4*4 chessboard icon. Cut & Paste a PGN or FEN.

You posted FEN for the starting position