In the factory manuals it is referred to as a Vacuum Pump Booster part# 2233295 with a VAC marked switch like the heater switch. It is listed in the GMC manual for 47-53 parts in section number 23.3787 pg 450. It was a factory option to keep the wipers working under load condition when they normally stop.
Here are couple pictures of one I found mounted on a 1950 2-ton.

http://good-times.webshots.com/photo/2629558360102454676CIHAvy
http://good-times.webshots.com/photo/2379570540102454676YQZtbb