Must have worked loose. Then rattled around a while. Thinking about 7 and 8 have been jumpy on gravel or really uneven surfacesfor a week or so. So the thread wore out, I compensated with the barrel adjuster and today it just decided to fall off. Wasn't moving fast or it could have be very unpleasant. Got a new one that was 99% right and dremeled it to make it fit. Only needed a few mm here and there. Put the whole system back together, tightened everything as far as is safe and it rides nicely now. Still, given its a small and light part carrying an extra can't hurt.