according to the Model t article on the history of coils (google) the early ones had sides that were nailed along the perimeter, the newer ones slid in a groove and was held with two nails at the bottom.??
Yes, I believe 13 or 14 they switched over to the slider with the two nails, but they still retained the brass top, even though Ford never polished them, the were painted a flat black from new. Are you selling these or just buying to restore?