I've seen the pivoting forend latch typically used on Robusts referred to as a "Rigby latch". And at least a couple of the ones I've owned had the lump go through the floor of the receiver. One of them was marked No 28, just as James' gun is marked No 7.

I've got a book with a decent lexicon of French gun terms: "Les armes de chasse a canon lisse" by Pierre Fonteneau. The lexicon doesn't give the "crochets encastres" term, but there is a later illustration of barrel assembly in which "crochets" obviously refers to the lumps.