Don't discount the 'J' as an engravers mark.
I've seen much more blatant, larger and misplaced.

It may be the work of 'Joseph' (Joseph Joseph) Firearms engraver. Last I knew he was in Cody Wy.
He just goes by the one name,,seriously, he considers himself a citzen of the world.
Does engraving, painting, all sorts of arts. He signs his works 'Joseph' but a simple 'J might be appropriate on some pieces.
Diamonds, ivory, gold,,,that'd be something he'd do. But I'd expect better quality quite frankly.

Just a guess though that it's his work,,,