I think I now have a diagnosis (from a well-known JBL expert):
That's indeed a factory original K120. Here's what might be the problem: ... The foam filter under the foilcal is rotted and lots of pieces have migrated into the gap causing the rub. It'll be a challenge to clean it out without invasive work. I'm sure this is probably the culprit. Either the foilcal needs to be removed and cleaning from the rear vent hole, or the dome needs to be removed to attempt to clean the goo out of the gap. No guarantees it can be cleaned successfully. The second ebay photo shows the foam filter is rotted.
I'm not going to return the speaker. I believe the most likely cause of the issue is not that the seller misrepresented the speaker but that the knocking around during the shipping process caused the dislodging and migration. I try to apply the Golden Rule to gear sales and purchases. I wouldn't want someone to tell me over 2 months after buying one of my speakers that he finally tried it and found a problem so I'm not going to do that to another. I'm focusing now on whether I should try to have it cleaned out or recone it.