W W Greener mentioned this Not for Ball & Choke marking thusly;
Prior to the adoption of choke boring most round balls fired from a shotgun had a close fit to the bore. When choke boring became common this was obviously not a good idea so the proof hoses began marking "Not for Ball" on choke bored bbls to indicate this. They could however be fired in complete safety "IF" the ball was undersized enough to pass easily through the choke. Many shooters however felt that firing any solid ball load would put them in an Out of Proof situation. The decision was thus made to rectify this by changing the mark to read "Choke" rather than "Not for Ball".
In the Modern Shotgun, Burrard indicated it was a rather common practise for British gunmakers to do the final pattern adjustments on a gun "Made to Order" "After" final proof had been done. This could be accomplished by either adjusting the chokes or relieving the forward portion of the bores. As long as the first 9" of bore was not enlarged it apparently did not take them "Out of Proof". My impression was this was done by the gunmakers themselves.
Of course being from the US all I can really say is what I have read from English writers, obviously no such situation existed here.
