You should listen to me. No, seriously, listen to me.
Join Date: Oct 2008
Location: Santa Barbara
Motorcycle: Blue 2006 GSX-R1000
Re: What gearbox is this?
I'm not sure I understand your question. Don't the ratios strongly suggest that it's the Yosh 320-506-0000 wide ratio transmission? As I understand it, there's an "A" mark on it somewhere, possibly on each gear. Or are you trying to determine who made them for Yosh? Was anyone else independently making gixxer transmissions? VERY pricey.
FWIW: My analysis indicates that the K5/K8 OEM transmission would have smoother spreads if 1st gear was 42/16. But that's unlikely to ever be available.