Fount in rec.games.chess.misc, I don't know the answer.
View Board
Puzzle: Mate in 6 - Black to move
solution
rot13
1... Dku3+
n)2. Aku3 t4+ 3. Xt3 Akr2+ 4. Xt2 tku3+ 5. Xu1 Et1+ 6. Ekt1 Ekt1#
---------------------------------------------------------As2 6. Eks2 Et1#
-----------------3. Xt2 tku3+ 4. Xs3 Ot4 5. Xt3 Akr2#
----------------------------------------Et3+ 5. Akt3 Ot4#
-----------------------------------------------5. ukt3 Ot4#
---------------------------------4. Xu1 Et1+ 5. Ekt1 As2#
------------------------------------------------ 5. Akt1 Ekt1+ 6. Ekt1 As2#
o)2. At3 St4+ 3. Akt4 ukt4#
-----------t4+ 3. Akt4 Skt4#
1... Dku3+
n)2. Aku3 t4+ 3. Xt3 Akr2+ 4. Xt2 tku3+ 5. Xu1 Et1+ 6. Ekt1 Ekt1#
---------------------------------------------------------As2 6. Eks2 Et1#
-----------------3. Xt2 tku3+ 4. Xs3 Ot4 5. Xt3 Akr2#
----------------------------------------Et3+ 5. Akt3 Ot4#
-----------------------------------------------5. ukt3 Ot4#
---------------------------------4. Xu1 Et1+ 5. Ekt1 As2#
------------------------------------------------ 5. Akt1 Ekt1+ 6. Ekt1 As2#
o)2. At3 St4+ 3. Akt4 ukt4#
-----------t4+ 3. Akt4 Skt4#