The Colt detective special was derived from the Colt positive special in 1927and the starting serial number for that year was 331000 so the serial number you show of 3021 does not match up as a Colt detective special. Now if the serial number was 383021 then it was made in 1930.
In 1972.
1950
Made in 1951.
Colt Detective Special serial numbers do not end in two letters. If you are positive the last two are "RD", you need to call Colt.
about 1936 (I believe)
Around 1966
1977.
Was produced in 1960.(750000-776399)
I believe made about 1913
Around 1950.
You can either call Colt or check the sn tables at Proofhouse.com.
Try the sn tables at Proofhouse.com