I recently got my hands on two SGI Granite Keyboards. I like them a lot, but I'm having trouble with the USB-PS2 adapters as well. I have beige 'Cypress Semiconductor' (Product ID: 0x2324 Vendor ID: 0x04b4) adapter that mostly works, but occasionally keeps sending the last key pressed over and over again until I press another key. I also have the Blue Cube and a Belkin adapter (identical to the one pictured above) that is seen by the computer as internally the same as the Blue Cube, and both are almost non-functional with this keyboard. Many key presses are not registered and when they are, it's often the wrong key etc.
The SGI units I have are the 'Made in USA' on the outside 'Made in Korea' on the inside ones, with manufacturing date from mid-1995.
Computer is a 27" Core i7 iMac running OS X 10.7.3.
If anyone knows an adapter that will work reliably with these, I'd be appreciative.