Hey Definition!
Ok I am going to muddy the waters here a bit and who knows maybe give you and idea or two.
I have a website (check my profile) and you may want to look at the pages there, and do a bit of comparing really. The BOM is clearer on your headcup than on mine, I thought it was BCM actually, but clearly its BOM I see. Now to muddy the waters here, if this is a mark of the maker then all you have to do is figure out who was supplying Gitane with frames era 1967 to 1971 ish.I have a 1969 Gitane I am restoring and that will be the pages I am directing you too. By the looks of the pics you have it seems other than the frame, maybe the brakes too, otherwise is all newer materials to upgrade a bike. Keep us posted ok, love to watch the progress.
Tim