X it was
When I Xplained, and sent pic to Mr. Miller, I was told that it was and could have been, "Experimental," therefore the X.
It was a standard 2.88 open rear end that by every indication looked to be original to the car. Its really tough to fake 46 years of caked on grease and oil.
My local tiger gurus say they have seen others with an X. It was one of the major reasons I sold on the car as it was tarred as a "nice collection of parts," a rather harsh comment about anyone's car.
Let's just say Jensen guys were sloppy, it was the 60's and not the Jaguar line.