SR was surveillance/reconnaissance, if I recall.
If I had to guess, the M in M-21 stood for mothership, or something similar. The M-21 was another Blackbird variant, this one built to launch an unmanned drone designated the D-21(I'd bet money that D stood for drone).
Only 2 M-21s were built(And under 40 D-21s), and one was destroyed when the D-21 failed to launch correctly and crashed into the fuselage of the M-21 under it(if I recall, the accident killed both the pilot and the launch control officer*). The remaining M-21 was retired immediatly after the accident.
Dunno what the A in A-12 stood for. I meant perhaps the Mig-25 was built as an anti-surveillance plane(countering the threat of the A-12).
Ditto for U(from the U-2?)
* On looking the data up, both ejected safely and landed in the ocean, but the launch officer accidentally opened his helmet visor in and drowned.