I also tried using datatype string but it doesen't work. Meanwhile I have studies the log files more detailed and found the following behavior:
when defining the device skeletal and when updating the derived device (first approx. 25 lines in the log file V_STATUS is declared as string with values 51 resp. 52 as specified in the web interface. The entity is "Modul 433 MHz".
Later on, when using the toggle switch in the web interface the value has changed in the decription in the log file (see line 164 or 185) value has been changed to on/off. With this definition it seem that the mysensors-Plugin is feeded has been changed from the specified values.
My conclusion is that the change is taking place somewhere outside the mysensors-plugin.
By the way, if I change the type to string do I have to use quotation marks in the web interface?
Thanks a lot for your support.