as it may already be known, there is always going to be some differences between the models. A may have been produced but had flaws. therefore, not put into production. However, D was put into production because it may have been a better product outcome. but, with some good aspects that the A model may have had, when the A was put into mainstream production, perhaps it was a upgrade version to the original A model?
looking online, the mainstream production of the panther A is sometimes referred to as the A2. But, still doesnt explain why there is no B. but, also looking online, there were two prototypes which would take place for the A1, B, and/or C. perhaps this explains the weird model numbers?