I managed to decode some Feldhell tonight after scrabbling about for the document showing what various data modes look like. It’s a strange mode!
Unfortunately the Spanish station sending it could not receive me so no QSO, but now at least I know what to look for. It takes me back to when I was interested in telegraphy systems as a kid, some decades ago now, but I’d never seen it in any form of action – represented by computer now rather than the machinery of old. Mind you what’s to say the Spanish station does not have said machinery!
There’s a whole bunch of info over at https://www.nonstopsystems.com/radio/hellschreiber-function-operation.htm