The phrase "silver seatbelts" refers to the color of the seatbelts in a car. It means that the seatbelts are silver in color instead of the usual black or gray. Full definition
Inside, each model will feature black leather with contrasting silver stitching, as well as silver seatbelts and 50th Anniversary logos on the front seats and floor mats. (autoweek.com)
Silver seatbelts also set this Legacy apart from its conventional counterparts. (topspeed.com)
The interiors all are black with silver contrast stitching (not gold, even though is the golden anniversary), silver seatbelts and the 50th Anniversary logo embroidered on the front seats and the carpet floormats. (cars.com)