It’s the other way around. The A-12 predates the SR-71.
The SR was actually bigger than the -12 being about 6ft longer, carried about 1,000 pounds more payload, and more fuel.
But the cost for bigger size and weight was reduced speed (mach .35 slower) and a 10,000ft lower service ceiling.
Yes, I corrected that on another post.
In 1969 I was working for Hughes Aircraft Company and they had a YF-12 (is that the same as A-12?)bailed to them to test the AIM-47 missile. I got to sit in the back seat (not fly, just look). NASA had a couple at the time and crashed one so they took the one Hughes had.