I'm still a little confused and the internet isn't helping. Was the NFB stamp requirement instituted in both proof houses at the same time as "CHOKE" (11 Nov 1875), or did stamping the flats with "NFB" come first then a few months later "CHOKE" supplemented or complemented it? Larry's post above makes it seem as if NFB was already being stamped - then it was decided that "CHOKE" could be substituted for it.
Sorry for any confusion, Argo. Both marks were introduced at the same time, under the proof rules of 1875--published by both proof houses on Oct 11 of that year. So from then on--until subsequent changes in proof rules--all choke-bored barrels would have been marked NFB unless they had a recessed/jug choke, in which case they would have been marked CHOKE.
