If any of you have any literature on Maytag WP-600 top-loading portable dishwasher, especially parts lists, spec-sheets, brochures, I would appreciate seeing them here or please donate them to the Ephemera library.

Maytag was very late to the dishwasher game.  They started with the portable alone in 1966.  In 1969 they introduced the all-pushbutton WU600 which was an under-counter only model.  It wasn't until 1971 that they offered a full line of under-counter and convertible portables.

They must be rare machines. Don't recall ever seeing one. How reliable were they?

IMO (Opinion and Observation) the end of the reverse-rack dishwashers heralded the demise of the brand; I wasn't a fan of the switch to the non-center dial washers, but the brand remained a good value, overall, into the Eighties.

The figure-8 filter and heater configuration was on the built-in models; the portable had a slightly different configuration and for crystal and delicate stuff you could and would turn off the heating element which included the water-heating.

I can't speak to the hinge spring defects (although that sure sounds like my KitchenAid KD-2 that has monster springs), but that dishwasher has the best racking system of any machine I've ever seen. The capacity was unequalled and extremely flexible. You can load bowls, plates and drinking ware on any of the racks and everything gets clean.