The case and headstamp are identical to a couple of "punkin ball" loaded Remington 10s that came along with my great grandfather's Rem 1889, which also has tiny rifle (aftermarket, I think) sights and 2 7/8 chambers marked "MAGNUM" as well as one chokeless barrel for ball and the other marked "Not for Ball" and very full choked.
I don't have an early enough ammo catalog to show the very earliest factory loaded slugs, but I'd GUESS that this is one of them. It reminds me of some early Continental slugs and would date before the "Foster" slug became our "standard."
I wouldn't want to get in front of that thing!
