Thank you to all who assisted! I gathered that the second set of barrels would probably have been an addition to the gun and not original. I was not certain that it would have been possible to get an additional barrel with a matching serial number. will get on the request for a letter. What I found particularly interesting is that the fore grips for the barrels were not the same shape, size or design. They also have a different method of operation. Now, if I could only recall how this gun came to be in my possession! I believe it was purchased from an estate in CA back in the late 1970's or early 1980's.... I am looking for a receipt... Thank you, again to all who assisted!