The proof rules in force at the time of proof testing continue to be the rules by which the gun is checked for proof until re-proofed.
Larry has correctly identified that the Not For Ball mark's disappearance coincided with the introduction of the fractional proof sizes. Therefore a gun marked with Not For Ball is easily dated as being proofed before the fractional sizes were introduced and hence has double the proof range of later proof sizes.
However, if the gun didn't carry any choke the Not For Ball was not impressed which leaves you with the need to establish if the gun is prior or post 1887. This can be done by looking for the nominal chamber mark in a diamond which was introduced along with the fractions.
As a matter of interest, in the 1875-87 period if there is no Not For Ball mark but '12M 12B' (for instance) is marked on either flat it means that that barrel was given a recessed or jug choke as standard. Note that both bore sizes are usually the same but occasionally they will differ. It has been suggested that this was caused by a long liquid lunch or perhaps an excellent night previously!
