Probably tested with English number 6 shot at 270 pellets to the ounce. Could have been regulated for 1 ounce, 1 1/16th. ounce or 1 1/8th. ounce. (Time to get the pocket calculator out). Boss should have details on file. It may specify the powder used (now almost certainly obsolete) and of course it will have been in a paper roll crimp case with card and felt wads. Often the person ordering the gun would specify what cartridge load and pattern he would expect and Boss would regulate accordingly. Time to get on the pattern plate with as variety of ammunition.
Sorry Drew but as I understand it No. 6 shot is 270 to the ounce. Lagopus.....
