I'm going to try to address some of the questions/comments in this thread with my understanding of plating. It is possible (i.e., the technology exists) to only plate a portion of a piece of cast iron. Therefore, Griswold could have plated only the outside of their skillets if they went to enough trouble. The following are the ways that I understand a piece can be partially plated:
1.) Only partially submerge the piece in the electrolyte.
2.) Maintain a gas pocket that does not allow the electrolyte to wet the surface during the plating process. (Could possibily turn a skillet upside down in the plating bath and maintain this pocket during plating. I perceive this to be a tricky operation, since some of the gas pocket will likely go into solution during the process. Therefore some of the gas would have to be replenished.)
3.) Coat the surface that is not to be plated with an insulating polymer. We have these materials today, but I'm not aware of any available in the late 1800's. The electrolyte solution used in plating is a reasonably harsh environment, so most coatings will not stand up.
I have in my collection ERIE skillets that have plating on the inside. However, the plating appears removed on most skillets that have been used significantly since cooking acidic foods (like tomato based recipes) will remove the plating from the surface. The plating still most likely remains in the pores.
In summary, it is my belief that Griswold plated their skillets completely inside and out. This gave them the appearance and cooking surface objective to justify the 100 percent increase in price the customer paid for plated items. This is only my opinion and time may prove me wrong. A surface analysis in a metallurgical lab would answer this question.
Roy Meadows