Unlike the rest of the tube frames, they have a brake on the PTO sheave. I'm guessing that was required by some sort of federal regulation that came into effect in 1978 when they were produced.
Any way, the part number is 173 3074. It replaced the belt guide/guard that all of the other PTOs used. There is also a rod that actuates it, but if I need to replace it, I can probably fab that myself
Here are a couple of pages from the manuals that show it. In the parts diagram, it's #75. Plus, here's a pic of the PTO sheave on the G11 showing it, and the actuating rod.