I have seen round 15” and 8” plates but this is a 10” plate. First is this a legit sign? It looks legit to me but wanted opinions. Second what was a 10” sign used for? Is it a rare size? Thanks in advance

Looks just like mine. I say it's OK. I think these were used on lubesters.

Lubesters were 15 inch

So were there smaller lubesters that had a 10” sign or were they on pumps or used another way?

Only know for sure about 15" very well could have been used

All three signs are considered utility signs. The most common applications were on lubesters. The 10" version is the rarest of the three.

100 percent original hard to find