The 5B is for the V32 Large rear bumper gaurd, the 5O is for seat belt delete, this one is an overseas only option. The -31B is an FSO number, basically a special order number that required corporate approval, had non standard parts (COPO) or was part of a fleet order. It was part of the 31st FSO order for that year. The B may indicate the car was part of a second order of FSO 31, or it may be the second car (B car) of a single order.