A picture would really help. Otherwise, do you have to open the cylinder to see it? If so it's an inspectors mark.
If it's on the outside of the frame then it was more than likely applied by someone after it left the factory.
I'm stumped, hard to tell from the pic but if it was stamped after it left the factory the finish would be displaced and you would see some bare metal. If it was stamped before the finish then it was either by the factory or it's been refinished.
At this point I would ask over on the Colt forum. http://www.coltforum.com/
Hmmm . . . an inventory number perhaps for an agency?? As suggested - post it on the Colt site - perhaps someone over there can fill in the blanks? Interesting . . . . I hope you'll keep us posted on what you find out. Looks like a nice piece - congrats on your find!