If my memory is correct the "Not For Ball" marking means the gun predates 1887. In 1887 this mark was replaced by the word "Choke" with amount of choke not given. In either case a round ball can be used as long as the ball is smaller than the choke diameter. Round Balls which were a close fit to the bore had been commonly used in cylinder bored shotguns. One of these bore fitting balls should not be fired in a choked barrel.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra