Another mystery to me. I had those holes in mine. I noticed these plugs in Lawrence Wade's unrestored '55 and noticed another plug up higher on the firewall (not sure what that one would be for either). I got all the plugs I needed from a salvage yard.
Here's that area from the inside (attachment below):
Note the paint run going sideways. I found this on my tub after removing it and before blasting. Makes me think the tubs were stood up on their sides and painted - in a hurry.
I did find a reference in the Parts Manual under, GROUP 31-04 -- COWL, DASH AND INSTRUMENT PANELS,
listing a button plug for the governor control hole in the dash. To me, this is a different hole.