For instance, here's a T-34 that looks pretty normal at first sight. Well, it has a proper name, "Komsomolets Zabaykalya" (Transbaikal Komsomol Member), but so do many T-34s.
Ah, but look at the observation device! It is not the one that a model 1941 should have, but the simplified one from the model 1942 turret! This was not a one-off design. This post at shushpanzer-ru has a few other examples, along with pictures of what these observation devices are supposed to look like, for the uninitiated (the last two).