[oletrucks] Do you know what this is?

["Hanlon, Bill (ISS Houston)" <bill.hanlon@hp.com>]

I ran across a GMC instrument panel for sale on the dreaded online
auction site that had a part attached that I am not familiar with.
Wayne was kind enough to put a picture of it on his web site at
        http://www.chevytrucks.org/temp/billphoto.jpg
It is obviously attached to the back side of a speedometer on a 55.2-59
GMC.  Any idea what it is?

oletrucks is devoted to Chevy and GM trucks built between 1941 and 1959