Here are some blurry stills of LTC Abrams tank.
From the looks of it, it appears to be a pintle socket welded to the front of the armored periscope cover with a .30 caliber trunnion to hold the M1919 .30 machine gun. A periscope cover was later designed by ordnance to serve this function, presumably based on this modification.
Presumably the Ordnance version
Installed on a Pershing: