More specifically Pilot #1. Effectively the last American heavy tank to ever be built, and the ONLY heavy tank (unless you count the Pershing, but that tank is more often classified as a medium tank, especially post-WWII) of the Cold War era to be built in "large-ish" numbers. Armed with a 120mm M58, and protected by up to 10-inch thick armor in some places, the T43 (a.k.a. the M103) was the US response to the Soviet heavy tanks (namely the IS-3 and IS-8/T-10); unfortunately, while impressive in appearance and firepower, there were a number of flaws in the early models, most egregious being the underpowered engine and transmission, the cumbersome two-piece ammunition that made stowage difficult, and in general a rather poorly-designed interior layout for the crew. Most M103s were picked up by the USMC (something rather unexpected, given the Marine Corps are usually given the hand-me-downs of the Army in most cases), where they would be updated to M103A1s and later M103A2s. Despite improvements to the tank's mobility and rangefinding equipment, the tanks were not really ideal for the conflict which the Cold War was starting to shape itself into, and with the MBT concept evolving even further throughout the 60s and early 70s, the tactical/strategic value of heavy tanks had become unquestionably irrelevant. So, having never once fired a shot in anger, the M103s (including A1s and A2s) were taken out of service, the Army and Marine Corps favoring the more versatile and reliable Patton tanks (which, by this point in time, were of the M60 standard I believe).

And yeah, I know that technically this is just the prototype vehicle, but a lot of the kinks and quirks that the M103 had were no doubt also present in this vehicle. Not only that, but much of the tank's history is rather brief by comparison to a lot of the other vehicles we've previously seen. Maybe not "vintage lawnmower and tractor" brief, but brief enough to be summed up in just one paragraph or so.

Anyways, tiny tank destroyer is next on the chopping block. Yay.