@ Bodger and MalcolmT,
I think MalcolmT has resolved this with his information that "X" was used as a prefix by Triumph for experimental engines. That would explain why engine X 773 E has a low number, as well as the suffix "E" which was not original to Morgan +4 engines. So Chris Lawrence removed the original Vanguard engine from the flatrad DHC and probably junked it, and the car is now powered by one of the experimental TR engines that Lawrence used for engine development.
Very interesting.
