Did Sinclair ever make a 12" round plate? I just read that the only round pump signs were decals. Not actual porcelain plates. Is that correct?

Being fairly new to gas and oil signs, I've tried to find pictures of old pumps, stations, and signs sold through reputable auctions to use as a basis to judge signs I was interested in. I've seen Sinclair pumps with round plates attached. Plates just like the one I've attached here.

Would a plate/sign like the one attached below be a fake?

Is there a place that outlines the general specs for a real plate or sign? It would be awesome to have a resource that you can look up and tells you what sizes a real sign would be. what sizes a real sign will not be.

Any info would be greatly appreciated. Thank you!

$_12[1].JPG
12" Round H-C Plate