There are Pentium III Coppermines with Slot 1! The Intel 440BX shipset should support Slot 1 Pentium IIIs with 100 MHz FSB, so upgrading to a much faster P III up to about 1 GHz is a theoretical possibility.
I still have a spare Slot1 / Socket 370 adapter lying around.
Until the machine gave up I had a 230MHz PII (Slot1, IBM 300PL) running on E@H. And a 180MHz MIPS machine (not used now, no time left for minor architectures).
BM

Vintage & unusual Computers on Einstein@Home
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Could I kindly ask you to use an app version number for self-compiled Apps that's not in the range of version numbers currently in use for official or Beta Apps? I'd suggest the generic versions 400 or 500. If you want to distinguish app versions yourself, everything below 400 is unused (apart from my own experiments usually in the range 0.01-0.50).
BM
BM