The original purpose of the date code was to identify when their one year warranty started. Sometimes you will find tools with two date codes, because the dealer didn’t sell within the manufacture year and put a new mark on the tool, though I have a two date code ratchet from 1961/62, and have seen double marks in the 1950s.
When Snap-On instigated a lifetime warranty, date codes became superfluous, though they were still used. In the late 1970s and early 1980s, Snap-On stopped using date codes. 1979, 1980, 1981, and 1982 dates I have never seen. I do have a 1978 1/4” drive universal, and I have 1983 dated tools. The only real reason for them to continue using date codes is for internal quality control. Considering their dip in quality in the 1960s through the 1980s, that is probably a pretty good idea. When you find undated tools with the patent number, that is probably the era. Government tools were dated, though perhaps not all of them.