Bob, your point is exactly what got me thinking and wondering. At work I do some really non-sense sort of stuff just because I'm told to. Otherwise, left undone, no one would care or notice.
Since the 10K was built with the school market in mind, similar to the locking covers, I wondered if it was an approach at safety.
This lathe was built in 1983 according to the paperwork from the factory I have on it. Unless the instruction plate was not changed from early production when a leather belt may have been used, I would think that the nylon/rubber flat belt would have been a factory oem type thing.
Might it be something like a bias ply tire left sitting for some time that develops a bit of a flat spot, would the belt tend to get two places that each time they go around the drives would cause some sort of slight vibration?
I'm not trying to get out of loosening the belt, just trying to understand.