OK, I’ve found the answer to my own question.
It’s a DS102W from Printec DS.
According to this data sheet (for a slightly different variant with integrated finger mouse) it’s rubber dome (“Principle: Contact by a silicon switch pad with carbon contacts”).
https://www.wagnergmbh.de/wp-content/u ... -black.pdf
Double shot keycaps (data sheet; Printec website states “lasered keycaps”) IP65 protection, “3.25 mm travel keys with tactile feel”, operation force 0,65 N.
Connection is a bit hazy, “Bidirectional serial output in IBM format”, but there are variants with DIN-SubD9, PS/2 and USB. And since the one on EBay has a proprietary connector, your guess about it is as good as mine.