Undoubtedly the gun originally had 2.5" chambers in that time frame. If it still does, Gamebore imports 2.5" 20ga ammo via Clay Shooters Supply. Not cheap, but good ammo and fine for that gun as originally chambered. You may be able to scrounge up some other Brit 20ga ammo in 65 ot 67mm which would generally be fine, although my experience is any 67mm-marked ammo seems to be more powerful loads and less comfy on the shoulder.
If the chambers have been lengthened, but have sufficient thickness, then any 7/8oz load should be fine. I prefer to keep them to 1150 to 1200 fps in a light gun.
