Hans, that's an M1A2 improved commander's weapon station. Because of the CIT-V screen and assorted electronics, the M1/M1A1 commander's weapon station (CWS) with its power traverse was deleted and an "improved" weapons station (aka cupola) was added to the M1A2. That improved commander's weapon station can take either type of M2HB.
Notice how the commander is grabbing handgrips, almost like a bicycle's handlebars, rather than the .50's traditional wooden handles and butterflies.
The M1/M1A1 commander's weapon station had several issues that often put it out of action, either item would make the weapon inoperable which in turn would deadline the tank (yes, the commander's .50 would deadline a tank!).
The first issue was with the M10 charging handle. That fragile cable would fray right at the point where the cable came out and rubbed on a metal pully. Guaranteed to occur when you really needed the machine gun operable, like during gunnery.
The second issue was the brass gear that raised and lowered the machine gun's elevation. This problem only occurred when the blank firing adapter was attached during training like NTC or other maneuvers. The excess weight of the BFA on the end of the barrel would cause the weapon to "see saw" as the tank travelled cross country eventually wearing the brass elevation gear.
While the powered traverse was nice, it was hardly used and moved the cupola in a rather jerky motion. I was glad that the A2 changed that .50 mount. Probably the overall cost kept the Army from retrofitting all Abrams tanks to the ICWS. Just imaging replacing every single commander's cupola on every tank!
As far as the coax on the older tanks, when I was on the CEV and the M48A5, our coax was a 7.62mm M219 machine gun. I did a quick google search and found that it came into use in 1959. When I was on the M60A3TTS and M1-series tanks, our coax was the M240 7.62mm machine gun.
I do not know what an early M48/A1/A2/A3 or even earlier M60 or M60A1s used.
As to how the M48's spent brass dropped, I'm not sure. On the M48A5 I was on, we used externally mounted M60Ds with butterflies and an anti aircraft sight. On the CEV and M60A3TTS (and would be the same on the M60 or M60A1) I was on, the spent brass and links dropped out an opening on the underside of the cupola's mantlet. The casings and links just scattered around the top of the turret and spilled over the side of the tank.
I bet the environmentally aware, recycling folks probably thought it would be better to collect the brass on that ASV. Them dang links were always getting stuck in the grills and falling into impossible to reach spots. I bet my old A3 probably had brass links older than me stuck in various spots.