I haven't a clue. To the best of my recollection, I never mixed on an Auditronics desk, so I really don't know much about them other than what I'd been told.
Anyone I've ever known who had an Auditronics desk ( 2 or 3 guys over the years) always seemed to be modifying them to one degree or another, most typically with third party XFO's ( Jensen comes to mind ).

I can't say for sure of course, as my experience with those consoles was limited only to the few guys I knew who had them, but perhaps the reason they were modding them was because finding the original parts wasn't all that easy?
I don't even know how many 501's were made... Is the Auditronics company even still around?

Can I ask - and you don't have to answer if you don't want to - but is there a particular reason you decided to go with a desk that needed that much repair, and that was also difficult to get parts for?

I mean, even on a good day, older consoles will still have maintenance issues...but at least with desks by manufacturers like Neve, Amek, API, SSL, Trident, even Raindirk and Wheatsone, at least you can still get parts for them.

You might pay a lot of money for the part(s), LOL, but finding them isn't really too difficult.
I dunno, I'm just thinkin' out loud, I guess...

Have you thought about contacting a company like Vintage King? Those guys see and refurbish used stuff all the time:

http://vintageking.com/

Here's another one you might want to contact:

http://www.audioupgrades.com/

Good luck...

-d.