Don't know the mark, but did find some info (that you might already have).
There's a web site called
The Museum of Ceramics, located in East Liverpool.
One of the pages there lists the ceramic/pottery companies that have been located in East Liverpool over the years. (
Wikipedia says that there have been over 300 pottery factories in the area.)
At the end of the list is this acknowledgment:
"William C. Gates, Jr., who compiled the above list, serves as Curator of History for the Ohio Historical Society. He is the author of two books: The City of Hills and Kilns: Life and Work in East Liverpool, Ohio and, with Dana E. Ormerod, The East Liverpool, Ohio Pottery District: Identification of Manufacturers and Marks."
I bet that second book, if you can locate it, will help a great deal.