Most optics from the WWII era are of lower quality than modern optics, but they are still very usable. More often than not you will find clear, though not perfect parts. Some things to check on vintage scopes like this are that the reticle still functions smoothly and there is no significant rust on the ranging ring. Some other things to check on vintage scopes are that the condition is very good, there are light, if any, ring marks, and few normal use marks. Make sure the optics are clean, clear & bright, and that the lenses are as scratch free as possible. If everything works as it should, the scope can be a good purchase. Make sure the external adjustments work fine and that none of the cross hairs is bent or loose. You can expect to see some exterior signs of use, but again, as long as the optics and crosshairs are clear, your vintage scope should be good to go.
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Some old scopes are so simple that they have no crosshairs and no adjustments. Some have what are called "TV views", which are basically flattened circles that allow you to get a little bit more left and right viewing.
Weaver makes some of the best vintage rifle scopes in US history, and though they are simple (fine and simple crosshairs), they sometimes come with variable objectives and variable power settings too. If you end up buying a vintage scope, make sure that most, if any, of the wear and tear on it is simply storage wear. Also make sure that the glass is clear and the adjustments work well. Remember that even though some scopes are old, that does not necessarily make them cheap rifle scopes. These can cost a lot of money as they are rare and one of a kind.
After firing many rounds in a long shooting session, it is understandable to experience eye fatigue, especially if using vintage scopes which do not have modern amenities such as clarifying lenses and zooms and whatnot. You can tell if you have eye fatigue if you still have blurry vision after an extended period of time shooting.