Attached Link: http://www.bmgparts.com/quad.html

Well, I think your partly right. ;-) "The quad mount brownings were exclusively used by the navy." Well, I distinctly remember driving past one mounted on a 2 1/2 ton truck during my tour in Vietnam, and no ships in sight. ;-) I've heard the Navy quad fifties, just like the single 40 mm, was ex-army equipment. I think the true story was that the Halftrack towed the Quad Fifty, Mount M-45, per the link. I was sorta wondering how would it shoot over the sides of the half track, anyway. H'mmm lets see what else I can turn up: The 57 mm Gun Motor Carriage T48, the antitank gun (with shield) mounted similar to the howitzer you show. (This was a non-standardized issue of several hundred that went to the Brits). T12- M1897A4 (ex French) 75mm gun on M3 Half Track (this is actually what you show in your picture. The 105s were still around in my day, and I knew that wasn't one in your pic!) T19- The real M2A1 105 Howitzer on a M3 Halftrack, firing over the cab, also not standardized, but 324 were built.