Jot down the serial number and go to Colt's website and see what year it was made on their serial number lookup. GCNM MkIV Series 70 pistols were made from 1970 till about 1983 when the Series 80 mechanical firing pin safety was introduced. Generally the closer one gets to 1970 the more the pistol is worth given equal condition, box, etc. Below is the box my 1979 came in which I bought new in 1979.
Of course these guys have the collet or "finger" bushing and the roll pin in the Ellison rear sight was known to break after shooting for a time.