Dennis,
Do you have the "blue book"? That has a section on Sidney Hollow Ware. The script SIDNEY is the older of the two markings with the block style "SIDNEY HOLLOW WARE CO." being newer but, I think, still before Wagner bought them. That I am not sure of however and I have wondered myself what Phillip Smith did and made after buying the company back. I think the history in the blue book leaves out some interesting and needed details. It's my belief that Wagner pans that are marked only "SIDNEY" on the bottom but are, otherwise, identical to pieces marked WAGNER SIDNEY are the ones made by Wagner in the Hollow Ware foundry. But, again, that may not be the real truth. I just don't have the answers you are looking for. I think your skillet is probably c. 1890's.
Steve