I've seen a number of places which state that there IS a manually aimed .50 ventral MG. I've been unable to find any photographic evidence though. From all of the pictures i've seen, there doesnt appear to be any ventral firing position.
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I've run into a number of sites with pilot bio's and photos, which clearly state there were two gunners, so, I expect we ARE missing a .50 MG in the ventral position.