Well, the carpeting used with the Parchment interiors was black, and we know the seat belts definitely weren't black on the Parchment cars - so there goes that theory...
Brian, you might want to try contacting Bob Perkins at Perkins Restoration, as his 10,500 mile, unrestored '67 Convertible just happens to have a Deluxe Parchment interior. It's hard to tell from the few photos I've seen of the interior of his car, but the seat belts look to be closer to the number 4 color option that Ssnake-Oyl offered you...
Good Luck!
Kevin