That does look like the glow plug relay. If you look at the starter from the side, you should normally see the starter solenoid attached to the top of it and wired. This is the same on the later 6.5L model gear reduction starters. Ford, I've heard has a solenoid separate to avoid damage to it from engine heat. I suppose if what you have in the picture is not actually the glow plug relay, it may be possibly a relay for the starter if the solenoid on the starter has no wire connected to its terminals.
I had to get my starter replaced and when I took it apart, the thing inside was worn down on one side and I'm surprised it even turned anymore. Replacing it with a gear reduction starter model for the 6.5L engines has paid off. I haven't had to replace it yet, though I had to get it overhauled because it was sometimes just clicking although one time in extreme cold, it actually wouldn't stop cranking. There is a fix for that on a cucv page, steel soldiers, I believe.