This is a white plastic roller assembly.
Used by Elco, and Better-Bilt aluminum sliding windowsFlat edge design3/8 in. Brass Flat-Edge Roller Assemblies (2-pack). rollerWhite plastic design with brass roller2-packInstall 2 per windowA slotted head screwdriver is required for replacementFor further dimensional information, please see the line drawing 3/8 in.