I have a pair of 1917 Illinois license plates. Something I found interesting is one has vertical slotted holes in between the numbers and the other plate does not. I am wondering if the plate with the vertical slotted holes is in case the plate is mounted on the radiator so, some air goes through to cool the radiator? If anyone knows the reason for this I would appreciate it very much.
That's absolutely correct. Illinois used a ventilated plate for the front and a solid one for the back from 1912-1918. 1912 used one with holes drilled all over it, 1913 was a stencil and then the more familiar slots were started in 1914.
I have a pair of California pre-state issued plates that are solid and mounted on a screen for the front. I have seen other pre-1914 Californias that are similar, but I don't know of any other states that used a ventilated plate on the front.
Our givenors making license plates is an Illinois tradition. Currently there is two in prison, I think, with one waiting. One sold truck drivers licenses and the other was selling Obamas senate seat.