A pic would help,but this is what I remember. The 1218 is square, not squareish, but square. The 1220 is sorta square, almost like one took a regular skillet and sorta pushed in the 4 sides. The sides are rounded. The 1218 is older from around the 30's, the 1220 from the 60's, so not sisters but closer to mother and daughter. ;D. The lid for the 1218 is hard to find and is cast iron, I believe that 1220 could use the same glass square lid that came on the Wagner Benedict Pan.
Some Wagner's are unmarked, maybe to sell to hardware stores, etc.