As far as I know, this hole is open and was to stick your finger in to lift up the floorboard. I have a number of 1937-1940 GM trucks and they all have this open hole.


1939 GMC AC-102 Long Bed 1/2-Ton
In the Legacy Stovebolt Gallery
More details in his DITY Bay

1940 Chevy WB Series 1.5-Ton
In the Stovebolt Gallery