The "X" was just a way to tell a HD block from the regular. And the CX blocks are really thick. I had one and it had much thicker walls compared to the regular CJ block. Pop out a freeze plug and check the wall thickness using the drill bit test. Also look for the 428 cast inside. I been reading up on service blocks and that sure sounds like one. Look for a number with a couple of dots under it before the J16. Also the J is September not October. The foundry skipped the letter "I" in the date codes as it could be confused for a "1". Also no production blocks had the ribs only service blocks. Thtas why you see no casting or production number. The 66-427 is just the rear bulkhead of the 427 used in casting and it's on lots of other FE's besides. The sandscratch "C" was to designate the CJ block and was not a cast in letter like the "CX". The "A" blocks are the std 428 without the extra crank webbing. The best I can figure with the "CX" is it's to designate a CJ HD replacement block but that's just a guess. Checkout
www.428cobrajet.org and also an archive search on
www.fordfe.com for service blocks and you will find lots of info. Gotta run, G.