I was planning to make a custom Dovchenko in US army uniform figure and doing some online research on the army badges and found out some information.
The "A" badge on the left shoulders is the insignia of the Fifteenth US Army
, which was deactivated in 1946, well before the 1957 setting of KOTCS.
Not that the MPs would be aware that it's outdated anyway.
My guess is that the uniforms were WW2 surpluses that the Soviets picked up.