How to ID C6 year?

Where are the casting numbers for a C6. I need to get one built and have a few but not sure the year of the ones I have. According to most of the rebuild kits on the market, there is a year break in the late 70's, 76-77 I believe. Is there a major difference between them or does it even matter? This is for off highway use.

Thanks,
Dave S.

There is a round cast disc sort of thing adjacent to the shift linkage should have year + month

Thank you, I'll check mine out when I get home this evening.