I thought there was a similar thread here but couldn't find it.
Inspired by a post I saw on Facebook
Just what is this object.
I used one in the 60's and 70's as part of my work.
http://i1200.photobucket.com/albums/...pskis9udve.jpg
And a close up which may give you more info.
http://i1200.photobucket.com/albums/...psj506sk0q.jpg