That's the same rifling profile as my WR 16bore

In a discussion on that rifle I showed some balls I'd swaged through the bores.
Octagons!

The rib is marked to note a whitworth patent, and I think the forend hook is marked with a patent use number.

Get that rifle running or send it to me so that it can hang out with mine.
;^)

Cheers
Tinker