Near as I can figure is the "X" in the casting made it a service block. Basically, it didn't originate in a car, but sold or used as replacement. Don't take it as gospel, that's just the most common explanation I can find.
I don't think that's a service block and here's why....... 66 was the first year for the 428. The date code on your block is 5J14 which is 65-Sept-14th. As 66 production started in July-Aug 65 I'd say it's a pretty early 428 block.