Well the "A" on the back of the block was the way they marked them at the foundry. The "A" block was the std 428 block and the "C" was the CJ/SCJ. Although a few "A" blocks have been found in some CJ cars from the factory. If they were short on CJ blocks the next available one was the std 428 block. Supply and demand. Does the block have the double or triple webs at the crank supports? Check out this website www.428cobrajet.org you can compare the "A and C" blocks and see pics of each. Go to the component ID section. This will cover what you need to see and ID. You can also pop out the center freeze plug in the block and using a flashlight look for the"428" cast inside. Let us know what you find! G.