Good morning, Jaime;
The main difference between any of the lenses without the -X engraving and those with the -X, is the destination. The ROKKOR-X lenses were those shipped to North America and were imported by the Minolta Corporation here, and they carried the Minolta Corporation "M-tag Warranty." The only difference in how the lenses were treated while in Japan was that the -X lenses did have some minor cosmetic differences (such as the "-X") and they received some additional quality control checking requested by the Minolta Corporation. This information was given to me originally by my local Minolta Camera vendor back in the late 1960's, who also explained that the "X" was associated with "Export." At least, that is what I was told, and there have been corroborating statements in books seen in recent years.
Functionally, you will not see any difference in the lenses with or without the "-X." Others have mentioned that there are some differences in the MD lenses, and the earlier MC lenses, which are functionally different mechanically from the even earlier SR lenses, all of which use the same basic bayonet mount. The differences in those lenses are related mainly to the meter coupling systems. MC on the lens refers to "Meter Coupled," not to "Multi-Coated," and came out with the introduction of the SR-T series of cameras. The "MD" variants appeared with the additional meter coupling lug for use on the "X" series of cameras, especially the X-700 with its Program Mode. Even the earliest SR lenses were coated, but the coatings improved, and became Multi-Coatings as time went on, and they were introduced on the production line when it was felt that the improved coatings would help. There was no point where the improved coating process change occurred with a lens series model change, such as the MC to MD. If the new coating process was better, they put it in.
Optically, there were small incremental improvements in the prime lenses over the years, but they were pretty mature by the time of the introduction of the Minolta SLR camera back in 1958. Yes, Minolta does predate Nikon in that regard. The main area where we have seen real improvement in lens quality has been with the zoom lenses. The ready availability of computers to aid in lens design has produced the major improvements seen there.
You might notice a slight difference in the price of a lens with the "-X." Sometimes it will be a bit higher, but with the Minolta Corporation M-tag Warranty program being over now, there is no longer any really significant reason for any price difference.