WOW !, That's interesting. That "S" indicating it being a 15000lb. special order 1 1/2 ton is something special indeed. That's a "Rare" truck. On a regular, 1 1/2 ton trucks are rated at 14000lb., and are 4100, & 4400 series trucks. 6400 series trucks are usually 16000lb. rated 2 ton trucks.