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)
Was produced in 1960.(750000-776399)
I believe made about 1913
You can either call Colt or check the sn tables at Proofhouse.com.
Try the sn tables at Proofhouse.com