The location of the numbers makes me think it's not an original Colt guard, on a Colt he numbers would be entirely on the flat area in front of the guard. Yours looks to be from an 1861 Army model, or an 1858 Navy model, but my gut tells me it's a more modern reproduction. Having said that I pulled out my Italian made '58 and saw that my trigger guard isn't numbered at all. Also, (and I'm not entirely certain here) I believe that Colt stamped ".36 Cal." or ".44 Cal." on the left side of their guards. I know I've seen that on a lot of early ones, but I don't know if that was common practice on all models or if it was done over the entire production runs.
With all that being said, it's still possible that it's from a period copy of a colt gun, and a good clear pic of the numbers might help to be able to determine if they are in the right period "font". My thought though is that it's a 20th Century part from a reproduction. I'll have to check and see if any of he companies that made them, or are still making them stamped the guard like that.