The T48 Gun Motor Carriage was the combination of the common M3 Halftrack and the US-produced »M1« British 6pdr Mark V gun.
Between December 1942 and May 1943, the Diamond T Motor Car Company as one of the four main halftrack companies (aside from White, Autocar and International) produced 962 of these cars, intended for Britain.
But as it came out, the British Army had no use for the T48, so only 30 pieces were allocated.
The Soviet Union, as Lend-Lease recipient second only to Britain, received 650 cars and, knowing the gun from their popular Valentine Mk.IX tanks, immediately deployed them to their armour units as the »SU-57« still in 1943.
The halftracks successfully took part at the liberation of Rumania in late August 1944, and several samples were also supplied to the Polish troops in Russian service.

Source :  Hunnicut, »Halftracks«.