Darn we use to use those kind of Hub caps on over the Road trucks.
Except without that bolt thru the Middle.
Propar if I remember correct or were they bolt on type
I get it now instead of a black plug that was in them they use a bolt which would be better.
But the Black plug has a small weep hole in the front that keeps the pressure down and keeps the seal from blowing out,but I guess not needed on something that don't do 70 MPH on the Highway.
BTW that is a nice build.
Some closer pics would help also.
The weathering is what I would like to see better.