I think it is ligit. The Marshal production ended in 1956 in the 845,000 serial number range. This guns serial number dates it to 1956 as well. I believe that this is just another example of using up parts on hand and wasting nothing. Everything else about the gun looks correct. Unfortunately, only a Colt letter will tell for certain. The seller is a well known dealer in collector grade S&W firearms and has a excellent reputation.