OK, now we're getting somewhere. What I thought was a hex nut surrounded by a lock ring:



was actually all locked together and was the front of this:




Which is basically #34 in Christine's diagram with a hole in it for the overload switch. The parts guy at the dealer was kind enough to print off the proper diagram, it's #65 here:



The outermost O-ring was hard and shrunken, so I'm hopeful I have found my leak. Sadly, the dealer didn't have it in stock, and I missed the cutoff for tomorrows delivery by 10 minutes, so I'll have to wait till Tuesday.

Thanks again for the help, I'll update when I get the new O-rings installed.