Tachometer install for 1957 Bird
 

Hi,
The tach in my '57 is kind of wobbling around in its housing. It's incredibly hard to see up in there, but it looks like perhaps it is attached from the back to the housing by a single screw that draws it tight and away from the front glass. Does anyone know if this is true? I don't see a screw in there, but it's hard to see whether or not one belongs there. I'm guessing that it does. If so, any idea how long and what size thread?
Thanks!
Jim