This is the black nylon guide block that helps keep the sliding door roller on the center track. This looks similar to the early Vanagon, but it isn't. If you have a 63-79 VW Bus this part is for you.
Engine Compartment Hatch Seal for 72-79 VW Bus
Front Door to Body Seal (left) for 68-79 Bus
Front Door to Body Seal (right) for 68-79 Bus
Engine Door for 71-79 VW Bus
Front Door window Seal for 68-79 Bus
Windshield Seal for 68-79 Bus