Wow, thank you very much Snautzer! To be honest I really started to doubt whether there was any information available as there is hardly any information avialable about the instruments in the 266xx series.
I had already found some pictures which clarified the dial from the "Abstimmungsregler"...
Thanks for your reply CR, highly appreciated. The information I have so far is mainly based on what is available on this site. However it doesn't provide any information about the switch and the dial.
Pre-war German aircraft like the Heinkel He51 and Focke-Wulf FW56 had switches installed to operate the FuG VII "Bordfunkgeräte".
As far as I know these switches consisted of a tuning switch for transmitting (Robert Bosch Abstimmregler Gerätenummer Fl 26608 ) and a more or less identical switch...