Something arrived in the post today, so was busy testing this evening:
A Supermicro P3TDLE dual So370 FC-PGA2, so with Tualatin support - and yes, that's a PIII-S 1266 running there. Also has an ISA slot, but no AGP.
The previous owner obviously didn't have a clue what to do with it - it arrived with 4 DIMMs, but only one of them was registered ECC. After removing the three offending unbuffered beasties, it got further than beeping insultedly about RAM, but refused to give video output on the PCI VGA I used. Fortunately I had an ISA card, which worked, but at the slowest speed I have seen an x86 run. Just building up an ASCII screen in BIOS took the better part of a minute. It reminded me of my old ZX81. Checked the primary display option (I suspected it was 'absent') but that was normal (VGA). After I spent 15 minutes setting time & date correctly (yes, that slow...) and saved, it would recognise a PCI VGA, but weird stuff in any event. Once I had settings sorted out, it worked fine, booting straight into a Linux LiveCD. Only disappointment was with my other new acquisition, the SATA controller in the top 64b PCI slot. It has universal keying and I sort of hoped it would not just do 3.3V (which it did) but also 66MHz. No such luck. Still, that's earmarked for a low-profile i815EP system, so not really an issue.
In a bit of a twist about what to do with it. On the one hand a dual Tualatin system is much wow etc., but on the other I have no sensible reason to want one and actually would prefer a SiS635T-based Tualatin system, particularly one with an uATX motherboard (i.e. the Jetway 635CS - AFAIK the only uATX SiS635T board). So should I dig up a second PIII-S 1266 - or just sell the thing...? Enough to mill over tonight.