I might be the one who posted about the Sun machines.
I just bought a Sun 404 distributor tester this summer, and refurb'd it.
The first thing I used it on is the distributor in my wife's Corvair. I discovered several things:
The bushings are worn out.
The mechanical advance wasn't advancing.
The vacuum advance was faaaar too stiff and essentially wasn't advancing.
I don't have the car fired up yet, but we drove it with the last engine and that distributor for about 10,000 miles. It always fell on its nose at the upper rpm range, and I attributed it to points bounce, and being the limit of the engine.
My daily driver is a '61 Suburban with HEI distributor, I've got a '54 Buick, a '56 Chevy, and '73 Duster that I'm restoring, and I'm building a '57 Chevy gasser. Just for me, that machine will get a bunch of use.
And all my friends are gear heads, so I'm expecting my beer fridge to be full for the foreseeable future!
I lucked into mine, but if I was specifically shopping for one, I'd get the 505, as it has a coil tester and condenser tester. I get the feeling that the 404 was at home in the speed shops, while the 505 was found in service stations.
-Brad