Yes, there was a .50 in the mantlet in the coax position. It stuck out quite a bit further than the .30 would have. If you use the Academy aircraft type, with the cooling jacket running the length of the barrel, you want to have enough sticking out that you can just see the first of the holes in that cooling jacket. Take a look at Steve Zaloga's M4A3E8 in the Gallery on ML. This was a pretty common mod. to 4th Armored E8's when they did the uparmoring.
As for the fate of the tank, who knows? There are tanks ALL over the place painted like Abrams'....there's one in Germany I know of (an army friend took a photo for me), one at the Citadel whichever Carolina that's in (a friend took a photo for me on his honeymoon), and who knows how many others. So I wouldnt' go by were one is now in a museum.
Oh, and someone said that the other .50 was on a center mount at the rear of the turret, but Abrams' tank had the split cupola loader's hatch....making the .50 mounted on that as per it's usual location. Plus, an extra .30 in front of the commander's cupola. Incidentally, he probably didn't use this tank very long, as he was bumped up in rank/command around that time, taking over, if memory serves, the entire Combat Command (A or B, I can't remember), and so may not have still be zipping around in a tank at that point.
Rob