I don't think this is supposed to be a wool, winter overcoat. I
think, (although it is hard to tell because I don't know what the sculptor had in mind) it's supposed to be the GI raincoat. Judging by the color on the figure boxes artwork, that would be my assumption. The part that confuses me is the overlapping horizontal pleat on the back, I'm not sure if the GI Raincoat had this? IIRC, the wool greatcoat had several vertical pleats on the back.
Having said that, there are variances in the designs I'm sure, for instance the picture that Rob posted, the coat has an overlapping pleat on the shoulder, which I have seen before. I have also seen raincoats without the shoulder pleats, different button configurations, different style cuff belts on the sleeve ends etc.
I do think the color you have applied MP is a bit 'forest green' for the lack of a better description. I would go for a more pale color like the box artwork, or a more OD color, like the photo I have posted below.
Of course, these are merely my opinions. I am by no means all that well versed on GI clothing. I am following this thread however because my figure painting skills are absolutely atrocious. :S
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