That type cowl vent was used by Chevy from 1932 thru early 1955 with only one modification in 1947 (added a point to the front edge). As best I can discern they all drained through the firewall via a soon to rot out tube that extended forward and actually dropped water on the 6 cyl valve cover. The early valve covers had 4 vent slots on the upper surface so I expect some water made its way into the motor. More trivia. The original 1932 firewall was used thru the early 1955 trucks built before the '55 body change. I noticed that your cowl is outfitted with a pair of below the glass windshield wipers. Mine ('35) had one hand operated wiper more or less centered below the glass--I changed that!
Here is a pic of my cowl in the rust removal tank. When I reassembled things I welded the vent closed and filled the seam with urethane calk so it would still look original.
I modified the firewall to accept the turbo diesel so there really was no way to keep the cowl vent anyway.
In this pic you can nearly make out the original vent. Also the 3 wipers above the glass.
Just to complete the trivia, the Chevy truck cowls changed completely from mid '55-'59, '60-'66 and '67-91.
EDIT: I meant to ask this. Do any of you know when the vents on the side of the "A" pillars were eliminated? You can see them in my last picture. They have slide up/down baffles on the inside and drain directly down the pillar to the road.