Thanks Jeff, I do have standard red interior from the factory. I located this information on another post concerning a 1967 fastback:
There were two types of red used on the 2D interior.
Group 1 items (instrument panel, steering wheel and column, console, door panels, arm rest pads, kick panels) were red R & M #5773R.
Group 2 items (lower windshield molding, dash pad, windlace, windshield posts, sun visors, metal door surfaces, quarter trim panels, quarter vent panels, rear back seat panel, overhead strip) were dark red R & M #5732R. The seat belts and carpet were color keyed to group 2.
Would this explain why the seat belts were a darker red?