if a New Service model made about 1920
You can find out at Proofhouse.com
if double action revolver it was probably made about 1923
Proofhouse.com has Colt sn tables.
what is the manufacture date of a .38 police positive special serial number b 81
No way to answer without the caliber.
1927
Sorry but we would need the serial number to answer.
1920
1918
No published sn data
1958
1950