9 gauge = .803" bores. Of course this mark simply meant it would accept an .803" diameter plug gage, but not an .835" one, so could be anywhere between.
Normal full choke of a 12 gauge (.729") of this era was about .040" constriction, or 11% reduction of bore area. This same 11% reduction in an 8 gauge @ .835" would equal to .788 so the .79ish would not be at all excessive for the 8 gauge.
The "Not for Ball" marking confused a lot of people. These guns could be fired with ball as long as the ball was small enough to pass through the choke. Prior to choke boring most round balls used were close to bore diameter. If one fired a .010" under bore size ball through a barrel having some .030"-.040" choke a bulged or split muzzle was apt to occur. If on the other hand the ball was .010" under choke diameter then there was no problem.
