After looking at a lot of car brands and years, most show a center opening that hides all of the tire side and stops exactly right at the rim outer edge.
If that is true, then a 18.5" opening would stop at the outer edge of a 17" wheel. It would not be a small tire like 33/34 Ford (525/17?), or other entry level priced cars. Must be for a mid sized car around 33/34 in my opinion...based on pics, not facts
Still a nice find, in a matching pair. My car is the 525-550-18, so they won't work for me.