Did you get the new element from Marathon?
The Marathon Care and Use Guide page 12 states:
STANDARD REPLACEMENT ELEMENTS WILL NOT FIT THIS WATER HEATER!
The tank warranty may be voided if the wrong element is used.
I went down and looked at my Marathon. The element base appears to be brass as would be expected since the port in the WH is brass. The markings on the plastic surrounding the wire terminals are not like the normal standard elements. You would not want to use a standard element mounted in carbon steel when it is inserted into a brass port.