There are a couple of points to be made here, raised by previous posts: NOT FOR BALL is definately marked both on barrels and flats in individual examples. Whether investigation would show that the mark moved from one to the other at a certain time, I don't know, but I have seen plenty on both.
As regards dating, the NFB mark was introduced in 1875 for guns with 'significant choke' so a few thou constriction didn't necessarily get marked NFB. I would assume that the gun in the OP was made cylinder and bored out with the choke area retained, hence the choke, as suggested by some posts. The other mark that came in in 1887 was the nominal bore size in a diamond so if that is not there, the gun dates from pre-1887. If you look at all the examples shown, none have the diamond.
