Well... I can tell you what it is not...
It is not a KX125, KX250, or KX500 of any year.
Anything in the V800 range is early to mid Kawasaki as the V800's were discontinued on the dirt bikes around 1995.
Why did team green go to the 38mm?
Because the 97 KX250 used a 38mm Keihin Air Striker which is a superior carburetor in every way.
It will add 1 hp to the rear wheel on a completely stock KX500.. That is dyno confirmed.
The flow bench shows that it does just as well as a 41mm Airstriker for volume and velocity. That is also dyno confirmed...
As a bonus, it gives you more torque on the bottom end