During WWII, the govt put out a clever little bit of cardboard with the outer layer slightly larger than the inner. The outer layer was black with sideways slots in it; the inner was white. You squeezed the top and bottom between your thumb and forefinger. Squeeze and the white appeared between the slots. Let up and the white disappeared - it was like shuttered blinker-type Morse lamps the ships used. Talk about low tech - that worked. Today it would be some $500 gadget.
When I was in the Navy and was sitting around the messhall table b/sing, we had two radiomen who would communicate with each other by voicing dits and dahs. Drove us all crazy until the Chief of the Boat told them to quit or die.