Dog gone it, Hoverbug ... how'd you guess that so
quickly-??!! 
You're right. It's on display at the Seattle Museum of Flight in Tukwila, WA. With 80 flights and 123.9 flight hours, this aircraft is the least flown of the remaining Blackbirds. This M-21 was never used for D-21 launches, but mostly as a D-21 ferry vehicle and chase plane. It's one of two remaining Blackbirds that feature a black and bare metal paint scheme.
Good on ya!
TAKE IT AWAY, HOVERBUG!!!!!!!!!!!!