The "Not for Ball" mark was used between 1875 and 1887 in Britain, and the serial number chart on this site indicates a very late 1870's production date. 11b 12m shows that at the time it was proofed it had an 11 gauge (.753" or so) barrel with a 12 gauge (.729" or so) muzzle diameter meaning around .024" of choke. My guess is that it is probably chambered for one of the short 10 gauge shells.
There were so many grades of Greener at the time it's not possible to estimate possible value with just this info.