Could not find a find a BRE spoiler back in 1976, so I settled for a Interpart. I was told this was a knockoff from the BRE. So what's the difference.

Mine must be an Interpart too. I removed it a few days ago to have a chipped place fixed and painted. The underside was raw fiberglass and gel coat, unfinished at best. After a little research online the only difference I could tell was the BRE has a longer path for the brake cooling ducts.