Argo44: NOT FOR BALL was used on guns with some choke from 1875 to 1887. In 1887 the NFB stamp was stopped and CHOKE replaced it.
The reasoning behind this (as I understand it) was that by 1887 the Gunmakers Companies had come to the opinion that shooting ball through chokes was safe.
Don't forget that changes to the proof laws has to be made by act of Parliament so nothing could change overnight.