[Tijeras_Slim]

Some single seat SR-71s were built and designated Y-12s (CIA planes). There's one on display at an Air Museum in San Diego.

[Pukin Dog]

I live just north of San Diego in Mira Mesa. There were NO single seat SR-71's, but you are thinking of the A-12 interceptor which was to carry the precursor of the Phoenix missile used with Tomcats. It was the SR-71 that was also used by the CIA with high speed, high-definition cameras, not the A-12. The A-12 was slightly smaller and lighter than the SR-71 and carried a fire-control radar necessitating the round radome instead of the flatter nose of the SR-71.

[El Gato]

Some single seat SR-71s were built and designated Y-12s (CIA planes). There's one on display at an Air Museum in San Diego

That would be A-12 (there were A-11s as well, and YF-12s, but the Y in that case means prototype, even the YF-12s were two seaters. Had a fire control system and missle very similar to the F-14's AWG-9/Phoneix combo, in fact the latter were derived from the former. Similarly the SR-71 was derived from the A-11/12, rather than the other way around. In fact the SR should have been the RS, but Johnson screwed it up during the announcment of it's existance, and so SR it became and stayed.