In an attempt to save steel, space and weight, the U.S. military decided the replacement of the common metal cabs with soft cabs from late 1942 on.
Studebaker had been developping such cabs before and therefore was able to provide a solution which should become standard for all 2.5-ton trucks.
From the middle of December of 1942 prior to the beginning of March 1943 all "Studebaker" were equipped with the open cab without the doors and with the tarpaulin top. Total production was 4902 6x6 U-3 and an additional 5104 6x4 U-7.
Strange enough, almost all are assumed to have been sent to Russia, where the climate was far from suitable for trips in the "tropical" cabs. So, it comes as no surprise that the Red Army refused any open Studebakers from March 1943 on, so the company had to switch back to the former closed type.