Hello Pat
Ive pretty much stopped posting on these boards. I wouldnt have posted now but a good friend emailed asking me questions about your gun. It seems some people make these things there lives and do nothing but hammer other people. So I dont care what someone 3 guns or his 1913 reprint catalog has to say but here is what I see with your gun.
Your gun looks very typical for its serial number range. I only thing I cant explain is the one X on the frame. I believe it to be to early of a gun to be marked with the XX. As for the dogs on an F, this is very normal for this serial number range. There are alos some Fs with a bird on one side and a dog on the other. There are not alot of Fs with dogs but Ive seen 8 or 10 other guns around this number range with dogs. Your guns is a 2 piece cocker, standard opener, push button forend gun.
As for the gauge. I have done a spread sheet of measurments of my guns. Ive got 8ga, 10ga 12ga 14ga 16ga listed. (No 20 there to modern for me). I have small frame guns in 12ga 14ga and 16ga.
I looked at the measurments of your gun and compared them to my spread sheet. I have no question in my mind that your gun started out life as a 12ga gun.
If you have any coments or questions please email @ bobnoble@hughes.net As im done posting.