The skillet in the OP here has more in common with the one at lower left of pg. 11 of the RB as well as the one at upper right. This is pure speculation, but bear in mind that, around this time frame, Wagner had been producing "Sidney" brand skillets in the Sidney Hollow Ware foundry it had earlier acquired, and continued to do so, apparently, albeit in the Wagner plant, for some time after selling that same foundry back to its original owner. In those two RB skillets, you can see certain things. The lower left one has what appears to be an arc shaped area at 12 o'clock where an arc "WAGNER" may have been. The "SIDNEY" at upper right also has areas that imply earlier markings may have existed. Quite possibly, and again, speculation, the OP skillet may have been made from a pattern that saw previous duty as a "SIDNEY" (not SHW) pattern. Similarly, the skillet at upper right on RB page 11 may have been from a "WAGNER SIDNEY O." pattern that had its "WAGNER" and "O" removed in order to make "SIDNEY" (again, not SHW) skillets.