rollback,
You might find New England Custom Guns helpful. You should be able to identify the drilling more precisely to them. It is a little bit uncommon, while broken ones are not too uncommon. You didn't say which one is missing. In many drillings it is necessary to take the rifle's firing pin out before taking the thumb lever out, and reinstall it after the thumb lever. If the rifle firing pin is the one missing, maybe it was damaged in trying to disassemble the drilling, and not replaced then.
Mike
