Argo;
Does the gun in question indeed have any choke in either barrel? The Not for Ball mark was to be used only if the barrel had as much as .008" constriction, or essentially a choke. A cylinder or improved cylinder barrel would thus not be marked.
Also, it is a very good possibility that even if it is choked the "Proofer" simply failed to stamp the NFB mark. I think such goofs on the part of the proof houses are far more common than we like to think. This would be particularly evident near a change in the marks. Of course, the date the gun was proofed is not the date the gun was completed & ready to be sold.
