is where the M2 50 BMG machine gun would be mounted for conveys.
That appears to be a GMC, 2 1/2 ton (of course) rigged for water crossing(air breather). The .50 BMG mount was originally put in place to provide Anti-Aircraft cover for convoys.
I never saw (in 20 years) anybody actually manning that position looking into the sky for a Ruskie jet or NVA piper cub. In the South East Asian Olympiad,When mounted it was all about ground attack. And, by the by, that is a pretty exposed f---ing position for such valuable firepower. We finally wised-up and and started putting them in the trucks body with sand bags&armor. That was just before they brought Quad 50's and twin forties in for ground protection. My father who participated in the WWII(Europe) and Korea, basically said the same thing (the mount was of little help in defense of a convoy from air attack).
Sorry, charley this is not a GMC 2 1/2 ton but a M813 5 ton truck. I drove around in these for a 6 months before I got out of the Corp. Had a lot of fun driving these.