Many later ribbed 428 blocks do have an X. Some have CX. I think all of those are from airport luggage haulers, irrigation pump engines, etc. Not 391 trucks. No "428 core" blocks that I am aware of, or have ever seen have "105" on the front. I have seen a few 390 bore blocks with "428" cylinder cores. They always have small lettered "352" up front, and ribs at the pan rail. '73-4-5 dates. I think these are service replacement blocks for 361/391 but I don't think the original FT blocks had thick cores. I believe that near the end, all service blocks had 428 cores, and a very few of those were bored 4.050 for FT "service replacement" blocks for the 391s. I'll bet a few guys have pulled a 391 apart that had 428 cores, but my thinking is that it wouldn't have been the original block.
About half of the 15 or 20 "428" core 390s that I have had in my whole life came to me well used, with a Ford bushing in the distributor pilot. This makes me think that maybe.....just maybe....anyone who went to the dealer and bought a 390 block in the 73-up era may have gotten 428 water jackets without even knowing it. If only we could ask Carl Holbrook. He would have had the info on that.......and probably kept it to himself..........
