Block that receives a " message" and send string values over serial connection, to control hardware devices ( switch ), for example, via Arduino based controller ( see https://sourceforge.net/projects/ardu-switch/  )
Prerequisite:  pyserial
Build as usual for a Gnuradio block:
-   download and unpack
-   cd gr-message_to serial
-  mkdir build && cd build
-  cmake .. && make
- sudo make install && ldconfig
If successful this block is into OOT section... Enjoy ;-)