A-12 was the Lockheed company designation. Most aircraft have both manufacturer and military designations. The KC-135 was the Boeing 717 (a designation since reused for the DC-9). The P-51D-5-NA was the NA-109. Reportedly, the designation of the SR-71 was supposed to be RS-71 but Johnson flubbed the announcement. An SR-71 should be an antisubmarine modification of a recon aircraft, or something.
The weirdest designation ever is another Lockheed aircraft, the F-117. Despite assertions to the contrary, the Air Force did not secretly keep using the Century Series designations and employ it for the F-117. When the F-117 was still classified it was known as Program 117, and when they started printing manuals they had to call it something. While it is not really a Fighter but more of an Attack airplane, and they even flew A-7D's to train for F-117 missions, the head of TAC said, "The best Air Force fighter pilots will be flying this airplane and no fighter pilot would ever want to fly anything designed "A.""