Not counting the trainers, the A-12 was a single seat aircraft and the SR-71 was a two-seater. That's the easiest difference to spot, although there are others.

The books by Crickmore go into a lot of the details.

Here's the image from the link that sc_rufctr posted. The raw image was huge, so rather than linking it, I attached it. You can really see a huge difference in the silhouette of the nose of the 2 aircraft.
http://forums.pelicanparts.com/uploa...1578768356.jpg

Greets you as you walk in...

http://forums.pelicanparts.com/uploa...1578773882.jpg

Yes larger chines on the SR....

Also, count the windows... SR is a 2 person aircraft, Oxcart is a single. That's why the SR is longer in the nose.

I'd always assumed that the chines along the fuselage were mostly parallel, but seeing the comparison between the A12 and SR71 above, I now realize that the SR71 is wider at the front and tapers towards the rear. I'd always assumed that it looked like that because of perspective and the length of the nose of the plane. It looks like the A12 is mostly uniform (probably getting a little wider farther back).

Very interesting.

I'll probably be getting one of Crickmore's books to learn more about the plane itself.

CRickmore...

Steve belly solly long time.

Oops

Me love flied lice!!

Crickmore’s various books are good reads, some more than others, but if you want to get into the details of how these things were constructed, there are other books out there that are more detailed. This one is a good introduction:

Lockheed's SR-71 "Blackbird" Family -A-12, F-12, D-21, SR-71 -Aerofax, By Goodall and Miller.

thanks.