Hi Jerry,
Good eyes and wonderment. I have only a 129A egg skillet to look at and it is gated on the TOP edge of the left, far side away from the handle looking at the top of the pan. You have to look pretty carefully to see two gate marks about 1-1/4" long separated by 7/8" between them. I think the gates were very thin and carefull broken off. There is NO evidence of any grinding. Griswold did tumble many of their pieces with "stars" in a large drum to help smooth, polish, and remove casting sand. That may be why the gate marks are so very smooth and almost blend into the skillet's top edge. Note that the 129A pan has the later style handle of the 53 pans instead of the early style on the 129 egg skillets. I think you are correct about the 129A being the best cast of the egg pans.
I have seen pans of the same type that were gated on one side or the other side. I have often wondered, especially with very small pieces, if two or more pieces were cast as one and then separated at the gates when removed from the mold. It would make sense to me to cast two or three ashtrays or No.3 skillets, for instance, together to save in the number of molds and pouring. What do you think of this idea?
If you look at the earlier Griswold pans they are mostly gated at the top, outside edge of the pan but not all the way around. They may be ground at the 8 and 2 o'clock position or the 10 and 4 position; perhaps even at the 8 and 4 position. I don't have enough pieces of Griswold to tell all the possible different positions.
On later, possibly just the small tm pans and late ones at that, the gating seems to be at the top of the edge instead of just below the top and the gate is not always ground but smoothed off by tumbling. Some pieces appear to be ground smooth at the top of the edge though. Again, I don't have many Griswold skillets any more to verify this but am going by past observations. Let me know if I am erring on this.
I believe all 129 egg pans were made in Erie and that the 53 pans were probably made in Sidney. Have no proof of either theory.
Steve