Again, I'm not an expert, but I'll pass on the stuff I've heard.
I've heard that some of FIE's guns were made by Tanfoglio. Tanfoglio is located in the Gardone and Brescia area of Italy.....sound like a link to the "GB?"
I do know that one of the pics shows a proof mark for a Gardone & Brescia blackpowder gun.
Maybe, since there are several manufacturers in that particular region (or so I've heard), they each produced some (either complete guns or parts which were assembled at another shop) and they combined to "create" the GB gun?
I don't know.....just some thought.
Oh, and Fingers, I did some internet hunting to try to firm up the stuff I found, and although I didn't confirm or deny any of it, I did see a reference which stated that Numrich carried FIE parts.....FWIW.