really interesting frame, but I've never seen long spearpoint Nervex lugs that look like those (and they are very impressive, too). Clearly the BB shell is marked "nervex", and it's undoubtebly 68mm wide, tho it could be French or British threading (you may have to remove the BB unit to find that out). Until that threading is known I wouldn't rule out it's being British (but I'm sure no expert on bikes this old). I associate lug work like that with olden British builders.