Here's the latest.
The 56A replaced the 56 series box as a 4 speed at the end of 1200 production. From available evidence it looks like the 56A was first introduced as the 5 speed option towards the end of 1200 production as an option in the GX.

The four speed 56A & both versions of the 5 speed 56A all shared the same main gearbox case, which is what we see in this picture, so there is no way we can tell for sure just what lurks within from this photograph.

That boss protruding from the side near the letter 'A' is for a cross shaft that is used in the column shift linkage when this case is used for the 3 speed trans, however I believe that the 3 speed was discontinued at the end of 56 series production & if this is true, then this part is simply a carryover from the 56 series case from which it is derived.

If the 3 speed was indeed carried on, then this case serviced the 3, 4, & 5 speed variants.

EDIT

The diference with the 56A boxes is that they used a diferent, & stronger bearing to support the front of the cluster gear than the 56 series found in 1200's & 1000's.