GTD-21B Drone

Big thanks to *Kajm for finding this info. Turns out I misread the sign, so now I feel like an idiot. Anyways, the history lesson:
The GTD-21B is a Mach 3+ recon drone, designed to be launched from the back of an SR-71 for recon missions deep into enemy airspace. It carrys a single high-res photographic camera over a pre-planned flight path, then ejects the camera module for retrival before self-destructing. These can also be launched from B-52s (carried on the cruise missile hardpoint, see a pic here: [link] ).

The surviving drone either went to NASA for use in experiments or were placed in museums. This particular one is #534.