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.
418306
You will have to call Colt to find out.
1950
Made in 1951.
A Colt detective special with that serial # needs a letter either before or after it. Nothing that low before 1970
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.
Proofhouse.com has Colt sn tables. You can also call Colt and they will tell you.
about 1936 (I believe)
1977.
Around 1966
I believe made about 1913