Looks like a spacer that keeps the tray guides away from the sides of the cabinet.
Tom from SWISCO responded:
February 22, 2022
Hello! We sell a ton of spacers that work this same way, though I'm not sure exactly what size you need. Start by looking at our 32-339 and 32-352B, which seem the closest to my eye. I've also made a list of other potential candidates that you can review below.