It's hit and miss when you call Colt. Depends who answers the phone. I've gotten an answer from one person, called back the next day with the same question and someone else gave me a different answer. Most of who answers the phone at Colt do just that "hello thank you for calling Colt". They have some reference material and can look up serial numbers but those who answer the phone have no knowledge of the firearms especially the vintage stuff. The only to really tell is with a factory letter.