Need help with date codes 742 - 8 3/4 center section

I'm looking for a 742 center section and have seen many available. I would prefer to get one with a date code in mid to late 66 or Jan 67. I can't for the life of me figure out the date codes. Some have 3 or 4 numbers, some have number-letter-number-number. I have searched google for a definitive answer and have come up with the following:

7M13 = 1967-12-13 or YMDD with m=12 (no use of "I" in the code) from a website explaining
6031 = 1966-31st day of the year (my guess from other date code websites)
These two make sense

But what about:
6P17
5U22
232
M12 (741 case)

What am I missing