Its actually American. It was made in hopes to sell to the American Army. However the sale of it fell through after seeing the MKIV the British were using so effectivly. Lost to time other than the half-dozen pictures that remain as proof to its existance.
"The C. L. Best Tractor Company of San Leandro, California built two mock-up types on their CLB 75 tractor. One had a semi-cylindrical hull with a turret and the other was similar but the hull had flat surfaces. These two machines appeared at a Fourth of July celebration in San Francisco in 1917, in manoeuvres with units of the California National Guard."
From: AFV-G2 Magazine, Vol.5, Number 4 in 1975. By Baron Publishing