I have another update to the RPM porcelain mark, this is in our new Antiques and Collectibles Information Center. Hopefully this update will take care of any questions that should crop up about this backstamp.
It does indicate that one needs to closely evaluate the antique china piece in order to determine when it could have been created.
An old piece of china will show signs of wear whether it is utensil marks on the surface from years of eating from it or rough places on the rim from a plate hanger from being on display. The underside will also show signs of wear. Whereas a new piece will not have these age spots. You might have to get a magnifying glass out to really study the piece.