It's the CATTB M1 prototype. CATTB stands for Component Advanced Technology Test Bed. It's a prototype M1 to test a 140 - yes, ONE FORTY - millimeter main gun. There's a Leo2 prototype out there that also tested the 140mm gun. It's an old test bed, in the 1990s I think. Probably brought out to be rebuilt into an M1A2 SEP or reused to test technologies that will be found on future M1 versions or new tank altogether.
M1A2 SEP v3 will not be looking like that IIRC. The modifications to make a v3 wouldn't be "too drastic" IIRC. No idea if v3 includes an active protection system like Iron Fist on the Merkava.
EDITED TO ADD:
OOPS! It's not the CATTB apparently. CATTB has a completely different turret profile. Digging up an old thread at TN reveals that it's an ATAC (Advanced TAnk Cannon) systems demonstrator vehicle which mounts a 140mm gun and autoloader (because 140mm is quite BIG).
Take note the blocks on the turret face in the photos posted by Alpha. Those are NOT additional armour, but steel plates to simulate the WEIGHT of heavier armour, or to redistribute weight. Those steel plates have been found on other M1s such as the M1A2 prototype.
This is the Leo 2 140mm prototype:
140mm was conceived because they thought that they needed it against the next generation of Russian tanks. But improvements in 120mm APFSDS rounds, coupled with the weight and size of 140mm rounds (requiring an autoloader and less rounds vs. 120mm) meant that 140mm was not adopted.