Aviation stuff, junior spotter division:

Rat-55 is the one and only NT-43A, converted from a T-43 (Boeing 737) navigation trainer to measure and calibrate the radar (and probably other) signatures of various test aircraft.
There is a very similar civilian-registered 737 about which almost nothing is known.

The World’s Most Secretive 737 Just Showed Its Bizarre Self Over The Mojave Desert

When it comes to shy aircraft that we know exist, the Air Force’s wildly modified NT-43A flying signature measurement aircraft, known by its callsign “RAT 55,” really takes the cake. The nearly 50-year-old aircraft has no definitive home. It lives at either Area 51 or the Tonopah Test Range Airport and spends its very limited time in the air flying, usually in formation with stealthy aircraft, high over the desolate and sometimes inaccessible ranges that span large swathes of desert in California and Nevada. While online plane trackers have seen data associated with RAT 55 pop up here and there in recent months, nobody had spotted the bizarrely shaped test jet for what seemed like at least a couple of years, that is until now.