Bed rail wheel.
This wheel is commonly found on Pioneer series units with double sized beds built prior to 1997. The roller wheel sits at the front of the rail, and acts as guide for the bed extension system.
This sized wheel was only used on double sized bed extensions. It is found installed on our pre cut bed rail part number 4731B6291 found here
Measures 2-1/8 inches in diameter.
Part number 33352