Since Soviet uniforms and field gear are poorly documented in contrast to German and American militaria, I did have few references to go on.
True, a lot of good sources (including online) on the topic are in russian so many western researches are forced to rely on whatever they have left, considering RKKA ammunition is not the most popular topic out there.
However, one title I used, "Red Army Uniforms of World War Two" by Europa militaria does state that the snow suit, which was either cut as a greatcoat or two piece uniform, was worn over the basic field uniform (which in this case is the M35/36 gimnasterka). The book also features the model wearing the field gear on the outside, which I implied was for ease of access to his accoutrements.
Yes I know that book, by Anton Shalito and Ilya Savchenkov, and I guess that’s the page you're referring to (as it's the only winter camo set in the book ) -
http://img149.imageshack.us/img149/7121/russianwrongcamoqy9.jpg I've
highlighted some things for you
As you might know
gimnasterka m35, to which you're referring, a standard summer uniform, is just a simple shirt and in combination with thin winter camo (like in the game...) would be a suicide to wear even in -20c (not considering the real tem. of -35>)
Moreover, if you look at my scan and highlights it says "was worn over the basic field uniform and special winter clothing: Telogreika, sharovary, shapka yshanka, and valenki", yet, they dont have either on the picture
(but here is what it means -
http://www.rkka.ru/uniform/files/arm12.htm full set, sharovary (pants), telogreika (jacket), shapka (hat), and valenki (boots)
(see some larger war photos on the page).
But anyway, I don’t blame anyone, as the book is very basic and not the best source to rely on at all, which actually explains why they've used the pre-war ammunition belt over the camo again just to show what was (potentially?) included in the set, as you can see those things are not water-proof, but his camo trousers - are, that is another reason to have it all under the camo + you have two, large-enough cuts on both sides of the camo shirt for easy access, as I stated above.
edit: So yes there are could be exceptions, as in any case, but you're not likely to find them to this particular one