What is the difference between a Mossberg 500 and 500A?

The model 500 Mossberg is a particular style introduced in 1961 by O.F.Mossberg & Sons. It has undergone many style changes over the years, but 500A, B, C, or D are not style changes.

A model 500A is the 12 gauge version of the model 500. 500B is 16 gauge. 500C is 20 gauge and 500D is .410 (which is actually a caliber rather than a gauge.