I know the reason for the gatemark but approximately what years were these skillets made?
Dale, my understanding is this:
The oldest pieces will have a circular "sprue" mark on the underside of the piece. This technique was used until the mid-to late 1700s. (Most pieces were cast upside down, to avoid having an unsightly sprue or gate mark on the top of the piece).
The next generation of cast iron have a "gate" mark, which will look like a long thin line on the bottom of the piece. This is where the iron entered the mold, and the technique lasted from the mid-1700s to the late 1800s or so. Pieces made around 1875 to the present time have the iron enter the mold from the sides (usually in two places, sometimes opposite one another), which creates a smooth bottom.
Current techniques have the iron coming in from the side, so look for one or more places on the side of the iron with heavy grinding marks--that is where the iron entered the mold. The grinding was done to get rid of the excess metal from the pour.