Knowles Radio Hypnotic Crystal

The following was first posted six years ago but there has never been any
response. In the hope that
there are now some new folk out there who can help me, I want to try once
again to find the answer
to this mystery.

I have recently seen a device consisting of an approx. 4 cm diameter, hollow
disc, about 6 mm in
thickness, with a short (abt. 4 cm) arm in the plane of the disc.
The disc has a see-through plastic covering on each side so that the
contents are visible. The
inside is half filled with what appears to be a fine grain sand.
The rim of the disc has the words 'Knowles Radio Hypnotic Crystal' engraved
on both front and
back - there is no other writing.
The short arm extends into the disc and carries, half hidden in the sand but
visible through the
plastic, a short pointer with a small ball at its tip. The ball is at the
centre of the disc.
What is this device ?
Is it a spoof or did it have some valuable function in the long distant past
?
Any help to solve this mystery would be much appreciated.