[Learjockey1]

My source is High Performance Ford Engine Parts Interchange, SA Design Publication, SA 56 by George Ried. I got it from one of the book stores. On page 91, they reference FE blocks with an” HI” and a “check mark” on the back. On page 92 under the 428 section they show a block with the "A". A block with an “X” is high nickle and “CX” is an industrial 428. They also mention a myriad of machining variations, but use your eyes to visually recognize the features. The folks they reference as experts and contributors are the brothers Jeff Sneathen of SEMO Classic Mustangs and Bill Sneathan of Sneathan Enterprises in Cape Girardeau, Missouri.

They did not mention a XX, however a phone call may get you an answer.
No phone numbers were mentioned.