This is where I store my Powermaster Optimus Prime's head. As you can see, it's a really small space, and it'd be a far more snug fit if this head had wheels protruding from it and could fold out into a Roller buggy mode. I think that this space was designed to be just big enough to hold Roller, and not much else. It doesn't seem to be big enough to hold any of the carded 1988 Autobots (e.g. Throttlebots, Triggerbots etc.). The space doesn't do anything for the robot or base mode; it seems to serve no other purpose other than to hold Roller. And of course, it even has a ramp for Roller to roll up and down from.