No, short-wavelength UV light is so fussy that it won't even travel well through normal glass, so the windows on EPROMS are made from polished quartz rather than glass. Anything opaque, like a label, stops UV quite effectively, at least for the purpose we are talking about here. However, depending on when those ICs were programmed they could be starting to suffer from 'bit rot'. For this type of EPROM the method of changing the stored value in a bit from the default '1' to '0' is to 'push' a charge onto the gate of the FET which holds / remembers the stored bit state. The charge stays there for a very, very long time (decades) but not for ever, and eventually what will happen is that all the '0' bits will gradually change back to '1'.
If this is already happening all you can do is find the original code that is meant to be in the chip, erase the chip and reprogramme it and that will make it good for another 40-50 years, probably. If it isn't happening yet it is a good idea to put each chip in a programmer, read it and over-programme it with the same code. (No need to erase it if you are only 'refreshing' the code). This will reset the 'bit fade' time back to zero.
Do you have any microprocessor toys like maybe an Arduino Mega? You can probably find ready-baked Arduino Mega sketches for reading EPROMs, so you can read the code out and compare it with known good code. Programming them is a different matter as it needs a controlled high voltage source, probably about 12.5V for these types.
I'm sure there is also an ST program available which reads the contents of all of the EPROMs installed in the machine and saves them as files, the paradox being that the machine has to be working at least well enough to run software for this dodge to work.
Did I not read earlier that you have already tried swapping TOSes over at an earlier point with these machines? That would be the obvious thing to try if you suspect a bad TOS, provided one of your other machines also has a 6-chip TOS, ideally the same 'nationality' to avoid introducing more complications.