as your looking down the gun, the gauge should be stamped on the left barrel back by the stock.
No
Try gunstocksinc.com
Turn of the century.
I've got one!
If you are speaking of the single or double barrel guns, you can change barrels. However, you should have a gunsmith look at the other barrel to make sure it fits properly, most of them need a little hand fitting.
Barrel, stock,
the knickerbocker was made by crescent arms Look under Crescent Tryon firearms
I found a new unfinished one at: http://www.gun-parts.com/index.html/
Try gun shows, want ads, gun shops, google.
you don't
Try e-gunparts.com, Shotgun News, Gun List, do a web search on "shotgun stocks".