I'm a bit out of my swim lane here, since I'm a squarebody guy, but I'm not aware of any body side moldings available on a stepside.
The generic info packet in the GM Heritage Center archive
https://www.gmheritagecenter.com/doc...olet-Truck.pdf
shows a BX2 molding available on fleetside pickups and utilities. It also shows a B85 available separately or included as part of BX2.
The RPO consists of the three center alphnumerics; you may disregard the leading "4" and the last unknown characters for the purposes of this discussion (ie "4
B85AB").
Any chance your truck was originally a fleetside and the bed got swapped?
K