I did some fairly bizarre modem design for a downhole
well logging camera, transmitting power and digital video over 5 miles of coax. I can
probably answer any modulation questions you may have, or point you to the literature I
used in my research and design.
I think I can be pretty certain to say the Germans were doing nothing
really odd back then. I am more interested in exactly what kind of set
they were using to do the FSK modulation (and demodulation).
--
Will