For the most part (and this ISN'T a hard and fast rule), all 1967 and very early 1968 production Q-jets will be in the two digit (letter/number) format. (For example, C7 would be March, 1967.)
For 1968 and early 1969 production, Q-jet date codes will be in the four digit (Julian code) format. (For example, 0339 would be February 2nd, 1969.)
Later 1969 Q-jets will have either the two digit code (if it was built by Carter), or the four digit code (if it was built by Rochester).
Again though, exceptions to the above have been observed in numberous carburetors, so you can't always go by these "rules". Also note that after many, MANY years of rebuilds, parts have been replaced and exchanged with other carburetors, so the body you have may not necessarily be attached to its original base plate.