Okay, so. The first 2 digits of the serial number are 18. Swear to God. You say it goes up to 17, but I am looking at a photo I took with my cell phone so I could triple-check, and it's 18.
The bearings turn freely. There are no odors, and as luck would have it, my partner was in the basement at the time the belt came off and says there were no screeching/grinding/etc noises, just the motor whirring while obviously not turning the tub. We didn't try to put the old belt back on because we thought it was too damaged and would just bind up again:
As for history...we bought this machine a few months after moving into this house, so...around the end of 2005/beginning of 2006. We have had no trouble with it until a few months ago, the first time the belt slipped off. We don't do all that much laundry--2 adults, no kids, although we do have horses and dogs, so there's the occasional horse blanket, and our bedspread gets the pawprints washed off it often. The dogs are short-haired, btw. The only long hairs in this house are mine. We try to be good about not letting random stuff find its way into the washer, although I have found the odd button or whatever, and one time one of us washed a pair of pants with a deck screw left in the pocket (I assume, because I found it when I took all the clothes out). But that was all way before any of this happened.
Oh! One time I found some pieces of white plastic in with the clothes. That turned out to be some pieces from around the top of the drum--something must have gotten stuck up there between the top of the drum and the lip above it. But that was also a while ago. When the guy came to put the belt back on the first time, he noticed the chipped rim and just said be sure not to overload it, so we've been more careful since then. At the time that he came, we couldn't figure out what was wrong with the machine or how to figure out what was wrong (or even how to get the front off--I guess this is laughable around here, but we had no clue, having never seen it done!), hence not even trying to fix it ourselves. But if it's going to be a recurring theme, clearly we need to figure it out, and hopefully figure out what the actual problem is, since he just put the belt back on and didn't do anything else.
In the meantime, we have another problem, which is that the belt I ordered came today and doesn't fit
No idea why, since it's ostensibly the right part number, but it is 1/2" - 1" shorter than the old one. Although I guess if the old one is too stretched out from whatever has been going on down there, maybe that's why it came off...but I'm not sure we'll be able to get the smaller one on, since I don't see any mechanism to release/adjust the pulleys to allow the belt to go on, and this is definitely too small to just walk on.