For me, this is an over-use of that letter. X for eXperimental id different from X for future versions, or for commonality. For example, I understand to use 32x to refer to the family Airbus 318, 319, 320 and 321.
Also to call 73x any 737 version from 200 to 800 since they can be refered to as 732, 733, 734, etc, so the x stands for any number, and also to 74x
Same again with M8x, since MD80, MD82, MD83,..., MD88 & MD90 exist.
What I do not understand is why people in this forum refer to the possible future 747 as 747-400X. Presumabily, it will be not 747400anything, but 747-500. So, if 747-100=741, 747-200=742, etc, and generally any 747=74X why not to refer to that as 747-X or simply 74X? In the same case, A3xx is used because the figures following the number 3 in the future Airbus type are not defined (although they would probabily be 350, for continuing the series)
O have I misunderstood the question?