I was given a Seattle SE440BX-2 with a PII 350MHz and wanted to slow it down to run some benchmarks. Years ago I read about the B21 mod for going from 66MHz to 100MHz and decided to try it in reverse.

On the back of any Slot 1 board you'll see the pins for the slot on the back side. There are 4 columns of pins, the 11th pin up on the inside back column is pin21 - see attached image.

Soldered a wire to this and then ran it to a switch which was glued to the front of the board and the other end of the switch was soldered to a ground point. There was a hole in the motherboard making it easy to route the wires.

When the switch connects B21 to ground it forces the CPU to use 66MHz, this makes the P2-350 start up as a 233Mhz. Tried it with a P3-450 and it did not work and this only will slow down a P2 with a 100FSB, it will do nothing to speed up one with a 66MHz from the factory.

Thought I'd throw this out there if anyone wants to mess around with under-clocking a slot1 board. Want to compare a PII 233 with a PentiumMMX233, haven't found any benchmarks of that, everything I found was a 200 vs 233.