That's a speedometer control adaptor, it's vacuum actuated. The 2-speed rear ratio (HI or LO) is selected with the button on the gear shift lever which operates a vacuum valve mounted below the floor. This valve has a small port that goes to the adaptor to keep the speedometer calibrated when changing the rear axle from HI to LO.
If it came off of a TF truck it will more than likely have a ratio of 1.36:1.
Looks like it was made by Stewart-Warner and that p/n would be theirs.
Mike B
