Hello Richard, i'm told this is a heatsink, but i'm not sure what it's protecting if it's on the +ve feed. I guess you'll need to open the box, perhaps there's a large diode, can't think what else might be in a power feed. if it is a diode your multimeter will be able to read continuity if you reverse the leads. I've read that some of these radios can work on 6V, but i wouldn't think it was anything to do with that, i think the voltage is selected by using a bridging link or similar
If you're not aware there is quite a bit of detail on radios in the Technical Manual, link above left and then search for radios when you're on the tech manual main page
Let us know what you find