The Kubota B6000 is "reported" to be the only Kubota model number that was produced as both a US model and a grey model. If this is so, then I don't know how to tell the difference. I would guess by the labels on all of the levers (English or some other language...).
I have a Kubota B6000 sales brochure produced by Kubota USA, Compton California, indicating that at least some of them were US models. I can't find a date on the brochure.
Regardless, be aware that the B6000's rear PTO spins in the opposite direction of the "standard" PTO direction (clockwise as you look at it).