These were reproduced but I don't believe there were ever any porcelain ones? Someone else may have to chime in here but I believe that the originals were only made in a painted tin version. Perhaps this seller is not aware of what a porcelain sign looks like and is only describing it as such.
I recently bought one of these flanges and there was no way to discern it from an original as it looked perfect except for one fact, it had been clear coated! Now why would anyone clear coat an old sign that was in great shape? It was one of those things where I would be unable to enjoy the sign because I would always be wondering????? I returned it and will continue to look but in this case I would make sure you have a guarantee on originality and a return period! After you receive it, you can make your own determination by yourself or with the help of others!
Everything Cities Service
Specializing in old Gas Pumps