Meisterwerke

Bl. caps are a3xb2, g3xf2 & h4xg3. Wh. only cap is axb. For balance, the wPh2 had to promote on h8. Position unlocks by reconducting bNh4 on c1, thus freeing the wNb1 that can be unpromoted on h8. When the promoted wPh2 is reconducted past h4, we can unlock by retracting bPh4xg3.

Retract

-1. Ng6-h4 Rc1-d1
-2. Nf4-g6 R..
-3. Nd3-f4 R..
-4. Bh3-f1 Rd1-c1
-5. Nc1-d3 Nc3-b1
-6. Bf1-h3 Nd5-c3
-7. B.. Nf4-d5
-8. B.. Ng6-f4
-9. B.. Nh8-g6
-10. B.. h7-h8=N
-11. B.. h6-h7
-12. B.. h5-h6
-13. B.. h4-h5
-14. Bf1-h3 h2-h4
-15. h4xg3 etc.

Starting with a Wh. retraction would lead to an unsurmountable retro-opposition on h4 between wP and bB. It is not possible to swap side to move. Therefore Bl. played last and Wh. wins with 1. Nc3!! b1 2. Rxb1# (not Bl. with 1 ... Bg2# ??)