This is the American M1941 mounted waterbottle cover/carrier.
The mounted water bottle was issued to the cavalry and it differed to the standard water bottle cover by the addition of two horizontal strips of webbing on the back. The webbing strips reinforced the back when it was carried by mounted as the back had more wear and tear when being used by the mounted cavalry troops.
As with several other items equipment designed for use by mounted troops can be seen being worn by paratroopers in WW2.
The M9141 mounted water bottle (canteen) carrier is often called the Airborne water bottle although officially it was called the mounted canteen cover.
Colour OD #3 (tan/khaki).