I have owned & parted out over 100 AD trucks over the last 40 years & here is my experience with VIN plate attachment...
1947-1950 Chevy trucks used the machine hex head/clutch head screws.
1951-1955.1 Chevy trucks used rivets.
I have never seen a Chevy truck with factory phillips head screws.
The only wild card is the GMC trucks because in addition to the hex head/clutch head screws..... I have seen a few of those attached with factory phillips head/pan head sheetmetal screws.
As Brad mentioned....Using the incorrect or brand new shiny hardware to attach an original VIN plate is a sure way to raise suspicion of the validity of the truck with prospective buyers , motor vehicle departments & law enforcement.
I would agree on the GMC used Phillips pan head screws
