My understanding was that Apple never implemented 20MHz between CPU and slots. It's all 10 MHz.

Hmmm. the hardware dev. notes for the AV series states:

n All data transfers on NuBus are synchronized by a 10 MHz clock. An additional20 MHz clock supports burst transfers in cards that conform to the NuBus ’90

specification. This permits faster data transfers than are possible with earlier

NuBus designs.

Click to expand...

Which I find ambiguous.

The H. Dev. Notes for the NuBus PowerMacs doesn't even give that much. I know I read something more definiitive somewhere -- or I think I did. Perhaps in "Designing Cards and Drivers..."