The number A12 is to use with 120 film with will give you 12 frames of 6x6 pictures, A16 with 120 film will yield 16 shots of 645 pictures, A24 is to use 220 film yield 24 shots of 6x6 pictures.
Then A series film mags have four generations. Earliest been the one with an ISO dial at the back, then follow by the round windows film tab holder, then the square windows film tab holder and lastly the most desire mag which is the one with the dark slide holder.
You have to see there four screw mount on the last version with dark slide holder to be sure it the right version, as there people will glue a dark slide holder on the older mags that will made it look like it is the last version in order to get more money from you.