It appears it should have been 754 with rear belts installed.

Remove the belts from the car and soak them in GM upholstery cleaner, available from the dealer, or another ph-neutral non-bleaching cleaner. Change the solution frequently and wipe, but don't scrub, the belts. Scrubbing will raise the fibers. Rinse many times and air dry. Many cleaners will damage the fibers in the belts or stitching and weaken them, so use only mild solutions.