This is old wording from gunsmithing vernacular not found in common language. It must surely correspond with some oldish British terms, which I don't know. I will therefore concentrate on the meaning. Let's have a go :

- queue de bascule type écureuil = squirrel type upper receiver tang ;
- découpe à bavette sur bascule - queue de pontet façonnée = undercut receiver - sculptured triggerguard tang ;
- gravure personnalisée, découpe à bavette sur bascule, pontet à double cordon = custom engraving, undercut receiver, triggerguard with double ribbon ;
- capuche + oeillet façonné = sculptured hood + eyelet ;
- longuesse d'un superposé Eloge avec sulpture en option et oeillet façonné = forend from an "Eloge" o/u with optional sculpture and carved eyelet ;
- réservoir à cartouches = cartridge trap ;
- talon acier ajouré = skeletton butt plate.

I hope this will help.