Hmmmm. When I was in the Air Force the tail number was the last 3 digits of the serial number and the year produced was also on the tail in smaller letters.
Does NASA use the same numbering system?
“NASA 844, aka N844NA, aka, 61-7980, the final SR-71 built. Also, the very last SR-71 to fly, making its last flight Oct 9, 1999 at Edwards AFB. Yes, this is one of two SR-71s that had a civilian N number, the other being the 2 seat trainer, 956.”