Bear,
The square part is the crystal itself. the conductive strips connect to foils plated on either side of the cyrtal element. To repair, you will need to get conductive epoxy to reasttach to the foil. See:
http://www.mgchemicals.com/products/8331.htmlThere are several other products available.
It is expensive stuff in any case due to the silver component.
It is impossible to solder the leads as the salt melts at 75 C.
Mouser carries two Kobetone crystal microphonium elementiums, 25LM024 and 25LM022. The 024 is closest to the D104 element in size and frequency response. So much so, that you would think it was intended to be their version of same. It is a pretty rugged package and I bought two for insurance on my own D-104.
Crystal earpieces that came with crystal radios of the '50s also contain a Rochelle salt element. You might consider harvesting one if all else fails.