It MAY have 2-3/4, but a smith with a chamber gauge can tell you in 5 seconds. It could also be 2-1/2 or 2-9/16. In any event, being that old and in poor shape, a trip to a QUALIFIED smith to check for soundness is in order before firing.
You could try Gun parts (aka Numrich) for parts or www.doublegunshop.com
- lots of good folks who know double guns and may have some leads for parts
Also look here: http://www.shotgunworld.com/bbs/view...d329b731c03a26