W W Greener discussed this in his book/s. The consensus was, going along with what Small Bore has said, this marking in no way meant a solid ball could not be fired through the gun "BUT" that it must not be a Bore Size ball. An undersize ball which would pass through the choke was fine. The confusion generated over this mark was in fact what led to the switch of "Choke" being marked on the bbls.

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