Guys, I'm purchasing a rough 71 Cutlass Supreme that I have some suspicions could be an SX. How can I ID a real SX if it has no paperwork (other than a title) and the original motor and trans is gone....it does have the dual exhaust rear bumper with cutouts on it that looks like its original to the car...

 

Rear bumpers are incredibly easy to replace, so don't use that for identification purposes.

Quick easy things to check:
Look for SX emblems or the holes where they used to be on the front fenders.
Look at the tune up / emissions decal on the radiator (if it's still there after all this time) and verify 455 engine.

 

If it wasn't built in Lansing, there's a good chance that there might be a build sheet on top of the gas tank.

 

About the only way to prove the car is an SX is to have a 455 block or TH400 case with a VIN derivative that matches the car's VIN. The 455 and TH400 were only available in the Supreme with the Y79 SX package in 1971.

 

The only items, if original to the car, as most wouldn't know or care to add to the car, are the rear big block control arm braces that run from the front bolt of the rear lower control arm up to the cross member where the upper control arm mounts. If an SX hardtop, there are big block fender braces that attach to the holes in the fender just ahead of the hood hinges and run down behind the inner fender to the frame. Other than these braces and what others have pointed out, there are no other ways to verify an original SX car.

 

Of course, these braces can be bolted to any Cutlass Supreme, so not iron-clad "proof".