Here's a guess... I cant see where the third hole is but in some states comercial trucks were required to display a GVW plate like these.

http://farm9.staticflickr.com/8105/8624860330_8ec524f93a_c.jpg
http://farm9.staticflickr.com/8259/8623756665_b6c798376b_c.jpg
http://farm9.staticflickr.com/8400/8623756499_1c88b3b504_c.jpg
http://farm9.staticflickr.com/8394/8624860834_e2bc1aaee4_c.jpg

I've seen many versions of this type of tag. I have a 1/2T plate like the first one I linked to. It's too narrow to fit across the top of the license plate and looks like it might fit the two holes you've marked in the pic.

1951 Chevy 3100
In the Stovebolt Gallery
www.rustyfiftyone.com