Reading from the rules of 1875, I don't find any minimum amount of choke required for barrels that were to be marked NFB. All of them were to be so marked, the only exception being those with recessed chokes. On those, the word "choke" was substituted for NFB. (And that's the first time the word "choke" appeared as a proofmark--although with a very different meaning that it had under later proof rules.)
Under the rules of 1887, "choke" replaced NFB on all choke-bored barrels, finally abolishing "not for ball". The exact meaning of "choke", however, was changed again under the rules of 1904. Under those rules, barrels with a constriction of .008 or more HAD to be marked "choke". But they could--at the request of the sender--be marked "choke" if they had at least .004 constriction.
So once again our British cousins confuse us with 3 different meanings for the "choke" proofmark . . . depending on when the gun was made.
