Found the piece from Manton's 1925 catalog. It is the Nitro Paradox between the Paradox and Fauneta rounds.

http://www.longrangehunting.com/forums/f...692/index4.html

Tough to see here, but the slug is hollow with a metal base.

I think by the 20's everyone's ball & shot guns were made under some form of the expired Holland patent. No more invisible rifling, oval bores, etc.