I finally tested a Seasonic SSH-350G 350W AT power supply that I got from eBay about a year ago. Wasn't able to test it before because of not having a switch that fit the size of the PSU's blade terminals.
Anyway, the PSU works great. I left it running for about 5 hours and there was only a negligible shift in voltage readings over that time. I tried to measure ripple using mVAC rms on a multimeter, not sure how valid that is but those readings looked good also, especially on 5V.
Initially I only loaded the PSU with a hard drive, and it looked like the 12V rail might need adjustment, but as soon as I added the load of a motherboard then it came perfectly into line.
The date codes show this PSU was made in late 1997 or in 1998, so it would have been late for an AT power supply. It's also a top notch brand and has a high power capacity for AT (350W), so it would have been pretty expensive. It was completely dust free inside, so I suspect it was never used. It may have sat on a shelf somewhere as a backup for an older server, or as an expensive retail PSU that nobody in 1998+ was going to buy. I'm thrilled to have it, couldn't ask for a better PSU for any AT system I'd ever imagine building. After 20 years of idleness I'm glad nothing popped when I turned it on.
I was amused to stumble across this THG page where somebody nearly bought one of these for a Core2. Presumably there was a photo of the specs label because somebody criticized the amps on 12V. Nobody noticed the total lack of any amps on 3.3V or any of the other clues that it's not ATX.
http://www.tomshardware.com/answers/id-215882 … adeon-5450.html
The eBay link showed 8 sold for only $15, I wonder how many of those went to people who didn't know what it was and couldn't use it.