It's a Quad .50's :

This must be an earlier picture (the rubber side skirts and MG mount are still fitted)
On a side note, here's another interesting M548-based SPAAG, the XM166 (fitted with the twin 40mm turret from a M42) :

Variant (driver's hatch) :


Trial footage :
https://www.youtube.com/watch?v=--pi-IOeBxEH.P.