Here's what I THINK I know. The very first model frames that had the sterling silver butt cap and no adjustment in the trigger for overtravel will only accept barrels with flat bottom lugs. I believe these are known as Type 1 through Type 4 barrels, depending on the markings and the mechanism for retaining the locking lugs. You have to go back pretty far to find one of these frames. I don't know the number when things changed, but I believe it's certainly below 25,000 around 1970 and know it's below 42,xxx because I have one in that range and it has the overtravel adjustment. Frames that have the trigger guard that takes the screw in the back for trigger overtravel adjustment will take all barrels, but you MAY have to change the locking lugs. Some of them (especially the "easy open" frames?) may want the two piece lug. I'm open to correction if wrong!