Colt Diamondbacks were produced between 1966 & 1988. Serial numbers started with the letter D between the years 1966-1976. In 1976-1977 started with letter N .In 1978 started with letter R, so my quess is, it must have been produced sometime between 1978 & 1988.
You will have to call Colt.
made about 1972
You will have to call Colt. Published sn data does not show any Diamondback with a sn starting with a P
Proofhouse.com has Colt sn tables. You can also call Colt and they will tell you.
made about 1977