Quick question:

Looks like an original '66 GT gas cap. Hard to tell from the picture, but it doesn't look to be in great shape, probably everyday driver shape. It'd be one that I'd take a toothbrush and clean up pretty good then fill the letters in with model paint and call it good as far as the outside. It's missing the bail wire on the inside and the gasket looks shot, step on it with a full tank and you'll have gas running down your valance. Probably worth about $10-$15 tops.

probably original..but most of us have a half dozen assorted caps...i have a least 6