Given the electronics available during WW2 I'm not sure frequency hopping is that practical. And a system that uses a piano roll (and associated mechanical gubbins) to control the frequency hopping is also not a very practical solution. For instance, you're using the system for radio comms between brigade or division levels HQs (as the piano roll setup isn't gonna be that portable), how well can you keep the two piano rolls in sync? Bugger up and suddenly your transmitter and receiver are jumping frequencies at different times and your 'call' keeps cutting out...