The U headstamp (and its variations) was common up to the 1980’s, and it may or may not still be used on some types.
As noted, Super-X was originally a Western Cartridge Company trademark, now used by Winchester. Many earlier Winchester RF cartridges carried the H headstamp.
You should also include cartridge case lengths, as there are a number of different .22s (short, long rifle, WRF, Extra-long, WMR, and others.) and that could assist in establishing dates. e.g., the .22 Short came first, and the Long Rifle came about 30 years later (but the earlier .22 Long has the same case length as the .22 Long Rifle). The .22 WMR is much more recent.
Keep in mind that ammunition has a very long shelf life, and it’s easily possible someone could have fired 50 year-old (or even much older) ammunition last week, so the firing date determination from the headstamp is always very iffy.
Condition could also be significant, as depending on soil chemistry (such as pH) and climate, fired cases left on the ground or buried can weather rapidly, and even those that are not very old can become old-looking quickly. So just because they look old may mean absolutely nothing, as they could have been fired only a year ago.