Encoding of pawn promotions in the destination field:

Destination row     Promotion to
as on the board     piece type
       3                n
       4                b
       5                r
       6                q
(for both sides)

thus, for white         for black
a7-b6   Pa7xb8Q         a2-b6   Pa2xb1Q
b7-b5   Pb7-b8R         b2-b5   Pb2-b1R
c7-c4   Pc7-c8B         c2-c4   Pc2-c1B
d7-d3   Pd7-d8N         d2-d3   Pd2-d1N