I too am a bit confused, but last time I checked 1879 pre-dated 1887, thus I see no problem with the "Not for Ball" marking. That would I believe be the correct marking for an 1879 choke bored gun.
Also in 1879 a gun was not marked for chamber, so it cannot truly be determined what the original chamber was, only the bore.
Nowhere near the quality of a Greener, but I have an old Birmingham proofed W Richards which has 12ga chambers with bores marked 14 from this era. Both bbls are cyl bore so no M marking nor Not for Ball.
