This is the M-21 variant of the Blackbird. M stands for “mothership”, which is designed to carry the attached D-21 drone. While both the A-12 and the D-21 were successful programs, the combination using the M-21 ended in disaster, including the loss of the other M-21 mothership. The D-21 continued in service in Vietnam, but was carried by the B-52 bomber rather than a Blackbird. In this photo, the D-21 drone is positioned on top of the Blackbird between the vertical fins.