No way to tell caliber by sn. You will have to call Colt.
Colt data at Proofhouse.com
A Colt peacemaker with that serial number was made in 1974.
@1974
A Colt peacemaker .22 model with that serial number was made in 1971.
1971
1915
1873
It was made in 1971.
1920
The Colt frontier peacemaker .22 model with the L prefix was made in 1974.(L01001 to L25000)
about 1873...........................
Need caliber to answer.