A year or two back, I found a couple cheap upper door panels labeled 'G-Body' at a swap meet. I thought I had a bonanza as there is LITTLE Regal stuff at swap meets, and It looked like they'd match the interior in my project
They were pleated vinyl, had some plastic chrome piping, and a very different pull strap. When I got them home they were a perfect match/fit for the shape and contour of the Regal uppers but were a bad color match. Long story short I could have used and maybe redyed in a Regal color to match.... but they turned out to be from a Monte SS. So maybe that's what you have or what was used for a core for yours?
I've also seen some leather kits that have matching uppers and sail panels in back and wondered if they were leather or a close vinyl match substitute.
Oh and early NA Regal's had vinyl upholstery or a combination of vinyl & cloth. .That's my 2 cents.