Upper receiver with "C" and "H" or "M"

[kursk]

I have 2 colt SP1 uppers, both with the "C" and one with a "H" marking, the other with a "M" mark.

What does the "H" or "M" mark mean? I assume it is a date stamp?

Thanks in advance.

[shiva]

They are forge marks

H is for Harvey

M is for Martin Marietta

http://www.biggerhammer.net/ar15/uppers/

[pezboy]

CM is early 70s and CH is mid 70s to 80s.
Dustin