I got my 750F stitched back together.
After a close look at the tensioner, the cause of failure was unexpected.
At first it seemed like I had a case similar to PeWe, where the rubber disintegrated.
BUT it appears that the upper roller actually seized. So the chain was running along one side of the rubber, and eventually cracked it.
Yes, the rubber material was garbage. It was hardening, and there were cracks along the edge of the wheels.
After 1,000 miles on the original rebuild (09/2014), I did a tune up including a timing chain tension set. When I performed that, I remember that plunger snapped into place. Because it was probably pushing the metal portion of the roller directly on to the timing chain.
I thoroughly inspected the chain, and found no signs of wear.