This may have occurred during the period when Colt was scrambling to comply with British acceptance. British decided not to accept guns with Pall Mall address ( If memory serves!) and Colt was furiously trying to change or alter their barrels to meet new British compliance rules on the barrel markings. in the maelstrom the serial stamping may have been overlooked and therefore a factory error. Lack of cylinder serial make have also been a result of this issue. Just my opinion.