Sammyboy is correct. There is no "The" Klappfahrrad in this world nor a company in Germany that is called Klappfahrrad. The word normally stands for a bike that folds with one hinge in the middle and that's it. Now the modern folders are called "Faltrad". Klappen is another word for falten (= to fold) ad an "r" and you get klappern =rattling. So there is a saying in Germany:
"Faltrad" comes from falten (=to fold)
"Klapprad" comes from klappern (=to clack, to rattle)

So - if you need a rattling bike or if you want your bones to rattle go and get it.

BTW: I am also interested in the obscure folders so for me its fine to have them posted here...but...just an idea: perhaps it would be a little bit more tidy to have one huge "what bike" tread and you add all your obscure findings there instead of starting a new thread for every finding?