SU57 if you are a Soviet. Never used in anger by Britain and America, the ones not supplied to the Soviet Union under the Lend-Lease agreement were converted back to standard M3A1 halftracks.
The Soviets designated it the SU57, it first saw action during the 1943 Dnepr River Offensive in the Ukraine with the 16th Separate Tank Destroyer Brigade, some units of this brigade went on to fight in the Berlin and Prague campaigns.
The model is a cut and shut from S and S Models using the Plastic Soldier Company M3, the crew in the front seats are the PSC crew with head swops from SandS, being a sad git I added shoulder boards with Miliput, the crew in the rear are AB Figures.
The conversion kit from S and S includes the M3 sprue, you do not need to buy the PSC box although I believe it is out of stock at the moment.