It would be in 36 caliber or 44 caliber...
No way to answer without the serial number.
You will have to call Colt with the serial number to find out.
what is the serial number ????
if your '51 navy is original (not repro) and no '0' in front of serial number, then made about 1853.................................. i have a 1851 also serial number 56870 could you tell me mine.at cortez@wildblue.net
frame.
@1851
1862
1867
The 1851 Colt navy with serial # 112343 was produced in 1861. (98000-117999) about 1861
made about 1852
It was made in 1974.
Need serial number to answer. Go to proofhouse.com to look up.