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"...but no one seems to know what the original stylus replacement is (should be something like a 1 mil for 33's, I think. I know the 3.0 mil I use for 78's isn't so good on vinyl). Anyone out there know what I should order from whom?"
I don't understand why you can't get straight info about this. The stylus that you want is a Stanton D50E Mk II. It is a .4 x .7 mil elliptical stylus that tracks at 1-2 grams (I'd use it at 1.5-2 grams).
This can be a fun cartridge. I used to be a Stanton dealer, and have a bunch of styli for these. The 500 was a workhorse broadcast cartridge dating from the 60s, with lots of different styli for particular uses. I like looking at all the different color codes on the finger grips. During the 80s, the model was dusted off and refitted with the Mark II styli, which have advanced magnet material in their cantelevers. Your model, in particular, was prized by a few audiophiles as a great "budget audiophile" cartridge. I wouldn't go that far, but I think that you may be very happy with this. And, best of all, I believe that your needle is still current with the manufacturer.
The two other styli in the series were the D50A MK II, a .7 mil conical, and the D50EE MK II, a .3 x .7 elliptical that should have sounded better, but the audiophiles seem to have agreed that the plain "E" had a better sound. Stantons are wonderful because you can mix and match your styli to your heart's content. You can even substitute Pickerings if you know what you are doing. And in the equivalent Pickering V-15 series, there's a "Stereohedron" (elliptical tip) that I can't wait to try just for the hell of it: the ultimate high-performance 500.
Let us know how this comes out, and how you like the sound. And let me know about your Thorens, a brand that I have repaired, but never lived with.