I agree with "elliotf".. In 1978, the Diamondback numbers were from R13901 / NA. The Diamondback,Cobra,Agent and Detective Special all shared the same serial numbering system and in 1979, they started a new number series. Starting in 1979, all four models started with serial # S01001 thru # S27028...............I hope I explained this right...........Per proofhouse.com R13901 started in 1978 and per http://www.coltforum.com/forums/colt-revolvers/12138-da-dates-manufacture-thru-1985-a.html S01001 started in 1979. I would say 1978 in answer to your question.
Thanks, but as many other colt firearms, the serial# does not show on their site.