These were originally made in the 1970's and are metal,not plastic.
Here's one,beat up a bit.
http://www.ebay.com/itm/332159661718?ul_noapp=true
The water slide decals are meant for the 1/4 glass window and are much smaller in size,6"X3"if I recall,and stand out like a sore thumb when applied to a standard license plate,which is much,much larger.