670 Heads date codes - help needed

[6.5liter.4spd]

There are some 670 heads with date code M216. I have read that the first letter is the month, then 2-digit for day, and last digit is year. However, M=13 and of course there aren't 13 months. Can someone explain this date code?

Thanks.

 

[melsg5]

M=December in 1967, they skipped one of the letters.

https://www.hagerty.com/articles-videos/articles/2017/11/27/identify-pontiac-engine-blocks

 

[6.5liter.4spd]

Perfect! Thank you.