The difference is key of insulator.
HIF3FCA-50PA-2.54DS(71) can mate with both
HIF3BB-50D-2.54R(63) (one central convex key) and
HIF3BA-50D-2.54R(63) (two convex keys).
HIF3FC-50PA-2.54DS(71) can mate with HIF3BB-50D-2.54R(63) (one central convex key)
but can not mate with HIF3BA-50D-2.54R(63) (two convex keys).