Idea is that it caught air from fan and road draft to give some propulsion to the ventilation system introduced in 1935...this is the "in" of the system, out is via a hole in a little lump by the pan. All non positive vent systems have problems at idle--this was an attempt to boost flow. Used into 1937, replaced by common round filter.
I customized one to work with my PCV system. I think they look cool. (this was a mock up...the vacuum pick will be moved to the plenum)