I've heard Frank Zappa had an instrumental album that was given a explicit content sticker?..maybe Hot Rats?

I remember reading about this somewhere a few years ago when i was binging out on Frank Zappa. I think it was Hot Rats(his first solo album) that was given the explicit content rating(it was almost completely instrumental except for willie the pimp which has a few lyrics. Of course it wouldnt have been given this rating until later on in his career when the censorship ratings were enacted. Im just trying to find a credible source for this info. This is extremely outrageous if true.