The threaded end of the pump is basically a molded-in bulkhead fitting. The nut would be a bulkhead fitting nut. Maybe that will help you find it. You may have to purchase the whole bulkhead fitting to get the nut you need though.
I called Attwood Corporation today and they were able to give me the part #914040. I still can’t find the part though. Looks like I will just have to buy the new pump, at least I’ll have a spare cartridge...
Update:
West Marine responded, $11.76 plus shipping and tax. 3 weeks to deliver. Too bad this part doesn’t share the threads as other thru-hull fittings. I’ll just buy a new one...