In trying to find an answer for you, I found this site which claims the first "in dash" option for a radio was the first Advance Design model in 1947. (look about half way down the page.)
http://antiquechevytrucks.com/Content/Orginal%20Restoration/advancedesign_main.htm This thread from 1/17 in this radio forum talks about radios being available as an "under the dash" option in the 41-early47 trucks.
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=42;t=000114
Your holes may have been drilled by a previous owner wanting some tunes or chatter as he went about his day.
If someone else doesn't chime in soon, you could contact this vendor. I bet they could answer your question. If you do contact them, post the answer, I'm interested in what they say.
http://www.redwoodradio.com/car_radio_repair.htm
