Robert - the letter/number code you are seeing is likely the one stamped on it at the Frankford arsenal during the conversion and sighting process. If I am correct, there may be the same letter/number stamped inside or outside the modified front barrel band. (they altered the band to add a higher front sight)

I know that this should be the case for the Federal conversions of this style - but I do not know if it is a correct marking for those done for New Jersey. If your's has an "NJ" marked on the barrel flat opposite the lock it would be one of these.

Hope this is of use - Gene (Who collects 1816 conversions)