“I have 8 Marlin 60’s”
Nobody can blame you for that. Sweet they are.
The sweet spot for Marlin 60’s is from 1980 to 1984, serial numbers starting with 20 through staring with 16. They used a serial number range that went down as the years went on from 1973-late 90’s. Subtracting the first two digits from 100 gives you the year of manufacture. 20 = 1980, 16 = 1984
The 80-84 models had the 22” barrel, full 18 round magazine and no last shot bolt hold open with the newer improved feedthroat. In 1985 they added the last shot bolt hold open, later they shorted the magazine to 14 rounds, and still later they shortened the barrel to 19”.