Colt 2nd gen 1851 navy serial lookup gives me 2 years back to back

I have a second gen colt 1851 navy with an early serial 60**. When looked up on colts serial finder it lists both 71 and 72 as the year produced. 71 says 1851 navy .36 cal, and 72 says model 1851 navy .36 second gen colt. Also, there is a letter O under the 4 digit serial on the frame. I'm curious if anyone has any input on this. I've scoured the web and all I can find is it's an early c series, but I've never seen another one with a letter under serial. Also, mine has a serial in a couple places most do not from my understanding, such as inside the loading lever and on the arbor. Any input would be greatly appreciated.

Actually, colt list the serial number as 3 guns, the first being an original 1851, but I know it's not.. lol.

Ok. I had seen 1 on an auction that was #68** with no letter. So It really is an interesting feature. Thank you very much for the info. I assumed it was the 72 make, which is what I registered it as.