Alternators-adjust my thinking

I must be thinking wrong-Help! This started when I wanted to pick up a 60A round back alternator for my '68 cuda resto. According to my parts info, from '66-'68, the 60A alternator carries the part #3000010 w/ pulley. Assumed that the seven digit number stamped on the back case by the large terminal would be the same. Now I am confused- can't locate this number and alternator numbers offered I can't locate in parts book. I must be doing something wrong, please straighten me out. How can I locate and identify a 60A round back alternator for early VR?