I am by no means an expert on this subject matter but from everything I have seen or read.... C=60s, D=70s
Somebody will correct me if I am wrong but if those heads were from 72 then the code should start 2D.

 

From what I found the first digit is the year and the next is the month, the last two are the day of the month.

So it would read like this
2 = 1972
D = June
29 = Day 29

 

http://www.mustangii.net/part_decode.asp
This is what I am basing my comment on unless that site is wrong.
Seems like the ID should start with a letter first and then a year. I know the site is for a Mustang II but pretty sure the same convention was used for the 60s mustangs.