The 1901 rear sight does have adjustments for windage. It has a double base, the upper part swivels horizontally on the lower.
Windage marks are scribed on the back of the upper base. The windage points are .040 inch apart.
The windage corrections are quite course, 5.86 MOA. (Thats .04 X sight radius divided by range equals windage corrections).
I have the type 4, 1901 sight which was used after serial number 330,000.
CPT USAR Ret
USAMU Sniper School Oct '78
Distinguished Rifle Badge 1071