Which one is sitting at the Air Zoo (Kalamazoo Aviation History Museum)? What variant is it?
$\begingroup$ Are you comparing the M-21 (an A-12 variant) in Seattle with the SR-71B in Michigan? $\endgroup$– Zach LiptonApr 6, 2018 at 6:47
The Air Zoo refers to it as an SR-71B. Their image of it shows it in NASA livery as NASA 831. This was originally USAF serial number 61-7956 which was one of two SR-71's built as trainers. 61-7957 was the other and it was destroyed in a crash in 1968. They were both built as SR-71B's, not A-12's or YF-12's.
Number 956 was transferred to NASA in 1990 when the AF retired the SR-71's the first time. NASA labeled it as #831.
There is only one other dual-control trainer and it is an SR-71C, serial number 61-7981 which replaced the destroyed #956. It was made from parts of the static test model SR-71 and the aft section of YF-12A #06934.
More info at Habu.org.
1$\begingroup$ I'm confused by your comment that there is only one other "two-seater". In The Blackbird Diaries almost every single story refers to the "back seat" and a second crew member. I believe that all the Blackbirds carried two crew members, but only the 71B and 71C could be piloted by the second crew, is that correct? $\endgroup$– abelenkyApr 6, 2018 at 14:12
3$\begingroup$ @abelenky You're correct. A limited number of two-seat, dual-control trainers were built, which is what Gerry is referring to. $\endgroup$– egidApr 6, 2018 at 14:26
$\begingroup$ @abelenky Your assumption is correct. I also found an image at the Air Zoo and confirmed the a/c number. I've updated the answer. $\endgroup$– GerryApr 6, 2018 at 16:38
$\begingroup$ @egid Thanks for the clarification. I've edited the answer to reduce ambiguity. $\endgroup$– GerryApr 6, 2018 at 16:39