Good morning, 5 String;
Minolta made several lenses of shorter focal length than 28mm. Back in the first half of the 1960's, when they came out with the SR-7 to replace the SR-3, they offered the 21mm lens with its external viewfinder. Why the viewfinder? Because the first 21mm lens was not of the modern retrofocus design (sort of a reversed telephoto, kind of), and its back element stuck into the path where the mirror would swing up. For that, they gave us the SR-7 with manual mirror lock-up and that external viewfinder. You can find the old 21mm lens every now and then, but getting the matching viewfinder can be a task.
Others that I can recall from memory include the 7.5mm circular fisheye, the 16mm full-frame fisheye, the 17mm rectilinear, and the 18mm, 20mm, and 24mm lenses. I like the 24mm for use with interior shots, but that is about as short as I like to go without special consideration to possible distortion effects. As you can see, there were several lenses shorter than 28mm.
As far as the "ROKKOR-X" lens designation is concerned, that "-X" part was something requested by the Minolta-USA people to go along with their "M-Tag Warranty Program." You might think of it as an official designation for a lens destined for "export" to North America. It may have helped with warranty claims for lenses not imported and sold by Minolta-USA. There were no real differences in the way that the lenses were built. They all were done on the same assembly line. The only possible difference (other than the obvious added engraving on the lens identification) was that the "-X" lenses for the M-Tag Warranty Program were to receive some additional quality control checking before packaging and shipping. Minolta lenses have always been good. The "-X" designation mainly meant that it had been really checked to make sure that it was good. They dropped the "-X" designation entirely in the 1980's.