I have wondered the same thing. I have noticed that the 63's (63 to at least 67) have the early features seen on pre-63 Russian AKM rifles. The 63 kits I have aquired all have the pistol grip type lower handguard, I guess these could have been replaced. The differences I am talking about are some minor lightening cuts that are not present on the early to mid 60s Romanians and the pre 63 Russian rifles. These are on the front sight block and gas block. Also the bolt carrier is a slab side style, does not have the step on the right hand side. Have also noticed that the 63 fixed stock trunnion is what is called a wasp waist instead of the single big hole. I am wondering if the 75 references is not just something one of the parts kit importers have recently came up with. Have you seen official references to a 63 and 75 designation?
jithaca