Some time ago (at least 10 years ago), I recall a discussion about the .50 cal mounted in front of the loader's hatch in 'Nam. I remember the consensus was that they repurposed .50 cal mount from M24 Chaffees and welded them to the top of the turret.
I did a quick search of the forums I normally haunt, but could not find the discussion. It might have been in a forum I no longer utilize though.
This is a Prime Portal photo of a Chaffee mount (on a Chaffee) that might give you an idea of scratchbuilding one or swiping one from a kit.