Thank you for posting the drawing. Actually, I've gotten a copy of the same pic and now have had a chance to get under the tractor and take a look-see. Turns out there's two of those keepers, on on each end of the pivot bolt. The cap screws on the keepers are kinda tough to get to, since the front keeper is behind the muffler and the rear keeper is obstructed by the electric clutch pulley for the mower deck drive.
At this point I just don't have the enthusiasm to wrestle with it, so it's going up for sale for parts, or hopefully to someone who can put it up on a lift to get to everything more easily. It's all in good shape otherwise.