According to Bart's 'The observers Fighting Vehicles directory WW2' revised addition he says for USSR trucks....
Makes and models...
GAZ-AA 1932-38 (1.5 ton 4X2)
GAZ-MM * 1938-48 (1.5 ton 4X2)
* = Based on GAZ-AA but more powerful M-1 engine (50bhp @ 2800rpm v's 40bhp @ 2200rpm)
It also says that other truck models in some cases only differed with the engine size other then that they were basicly the same vehicle.
Also in General data it says....
'Large scale production however started in 1932 with the GAZ-AA which was the Russian built American Ford Model AA truck of 1930.It was in production in modified form (GAZ-MM) and in many variations until after the war.'
So that we can be correct could you please open the bonnet (hood to you Americans) and tell us what sized engine is in the pictured truck? (LOL

)
I think we are both right but I have always known that pictured truck as the late war GAZ-AA
Hope this helps
Cheers
Cliff
PS> who can really tell for sure exactly when stocks of the smaller engine really did run out.