I picked these metal stamps up recently.Unfortunately the set is incomplete. They're made by Pryor of Sheffield, a company that started in 1849 as Edward Pryor & Son.
According to Sheffield History, as of 2007 the company was still family-owned. One person commented on that forum:
The Company is still up and running and lets hope it carries on for many more years. It was my daughters first job there and with any luck she will retire there. There's not many family run companies anymore , they all seem to have been took over by the giants more often American.
Below, how to use such stamps. This is taken from S.F. Krar, J.W. Oswald & J.E. St. Amand, Machine Tool Operations. (Gregg Division, McGraw-Hill Book Company, 1983).
BTW, this textbook is an excellent resource for hobby machinists like myself, with a wide variety of machine tool operations clearly described and illustrated with diagrams and photographs. All three authors had long careers in the trades before returning to teachers' college at the University of Toronto to achieve Specialist's Certificates in Machine Shop Practice. I'm pretty sure that such certification hasn't been offered for decades. I believe that high school shop classes today, where offered, are now taught by people who've simply graduated from teachers' college, with little to no previous experience in the trades. That's the result of the misguided educational policy of aiming to make every high school student graduate into a white-collar job, preferably IT. To misquote Herbert Hoover, "An Apple on every desktop and a mouse in every hand."
Below, the acknowledgements page from the book. How many of these machine and hand tool manufacturers are still around today, 30 years after the book was first published?