The VIN tag was welded to the body. The two holes in the tag and in the body were used to line the two up and hold the tag while it was being welded. No screws or rivets allowed.
My Canadian-built 1965 Valiant had the VIN plate drop off. Looked like it was glued in place as there no sign of welding. Chrysler used to make Cycleweld that was used to bond metal together chemically. They had a plant on Trenton Avenue in Detroit where they made adhesives and chemical products.