George W. Selsor owned, either in part or in whole, Germantown Tool Works (AKA Keystone Works) and Selsor, Cook & Co., the latter being known for well made coffee grinders.
Below is from an on-line auction catalog of antique tools, it refers to the 1867 patent.
SELSOR & COOK & CO./PHILADA. EARLY PATENT DBL. CLAW HAMMER PAT. MAR. 19, 1867 RARE MARKED EXAMPLE An EARLY PATENT DOUBLE CLAW HAMMER by Selsor, Cook & Company, Philadelphia, Pennsylvania, patented March 19, 1867 by George Selsor of Philadelphia. This boldly marked example is the best we have seen of this early patent. Something about this hammer seems to have encouraged the owners thereof to use the side as a maul and otherwise subject the head to abuse. This one escaped that fate. Nice. Auction: IC10-587 Estimate: $200.00 - $400.00
I have discovered other patents by George W. Selsor and at least one of his sons. On 3/29/1859, George W. Selsor filed patent No. 23,431 under his name and the names of J. Cook and W. Cook, titled, Improvement in Grinding-Mills.
I also discovered a general survey done by Hexamer General Surveys in 1877 of a manufacturing business owned by George Selsor & Co.. It was located at 38-44 Armat Street and Laurel Street in Germantown. They employed 27 men and 3 boys, and manufactured edge tools, hammers, coffee mills, and slaw cutters.
It is hard to say how long George Selsor was in business as a tool manufacturer. In looking for examples of items made by George Selsor or one of his companies, I have often seen comments that indicate he was not in business for a long time, even though people seem to agree he produced well made items. To get a clearer picture and more detailed information, I think I will need to take a trip to Philadelphia and do some hands-on research.