CH-53X will be newly built CH-53Es with upgraded components adn avionics. Externally will look like an E model, but will be all new internally. Here is more info on them.
http://www.globalsecurity.org/military/systems/aircraft/ch-53x.htm
http://hqinet001.hqmc.usmc.mil/p&r/concepts/2004/PDF/CP%2004%20Chap%204%20pdfs/CP04%20CHAP%204%20Aviation%20Combat%20Element%20-%20pp175_CH-53X%20PROGRAM.pdf
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