Thanks BlueFox. I suspected that might be the case. However, I was under the impression that the PDB-PT825-S8824 power distributor was the most recent version. Perhaps this is the most recent version for the old chassis? Do the newer 826's use a different power distributor with a different physical configuration? Also I found nothing in the manual indicating that the PWS-1K28P-SQ worked in SOME 826's but not others.
Supermicro documentation is usually very good, but this is not the first time I've found confusing or conflicting data. I guess that's what this forum is for, thank you very much. But I suspect that if I'd asked for clarification here before buying the wrong power supplies someone would have posted "Of course they'll work, read the effing manual."
No matter, I've decided to run PWS-501P-1R supplies anyway, my configuration doesn't need huge power supplies. The only reason I bought these was because they were the only "SQ" supplies I could find on short notice. But of the 6 501's I've bought for my 2 826 chassis, 3 have been faulty. One is completely dead, 2 trigger eternal "BEEEEEP" from the motherboard . . . unless i"m missing something with these as well. So at the moment, one server is happily running with 2 501's and the other is making do with one.
I just wanted to warn others to pay attention so they can avoid getting caught like I did.