I have a op I just got. It has the s/n, I think it's 833xxx, not near it at the moment, and then a "t" below that and a "s" below that. I read that the "t" may mean it was targeted at colt for poa/poi but not sure. I don't know what the "s" is. I am new to these, my first op but it was in nice shape with the box for the price.I don't know if it's a common mark? I appreciate any help.