Is there any difference between the distributor in an A15 and that in an A12? If so, how can I tell them apart?

There are a great many diferences between them as they can be made by diferent manufacturer, incorporate diferent design elements to accomodate diferent requirements, like smog emissions. They have diferent advance curves, even between the auto & manual trans versions in a given model. Some also have diferent vacuum canister advance characteristics. Some are electronic, while the rest are Kettering types [points]

However, starting with the A12 that was made from about '74 for the B210, they are "INTERCHANGEABLE" which is probably what you mean.
They are not directly interchangeable with the A12 engine used in the Datsun 1200 car & ute up to '73, so be carefull when you use the term "A12"

Next time that you are reading your service manual, have a look under the electrics section & check out the number of diferent distributor models that are used in the model of engine[s] that the book covers. Each diferent distributor model number is for a distributor with something about it that is diferent.

What, you don't have a service manual? Do yourself a favour,... get one.