Here's a snapshot from a 1929-46 Chevy Master Parts Book showing the cab details for a 1941-46 cab. The blue circle shows/confirms the A-pillar door bumper as mentioned by UtahYork and 46Sparky. There is no screw shown in the diagram in that above the rocker panel position as shown in 6321's 2nd picture above.
Perhaps someone installed that with a rubber bumper to fix a door rattle from compressed or missing bumpers that are shown in the cab rear door opening recess?
