G. Lunn, in the US the German company you are asking about is usually just called Walther. It was founded some time before 1914 and still exists today. You can find out a lot about it just by doing a properly spelled Google search, or looking it up on Wikipedia.
People here can almost certainly tell you about whatever Walther you have, but they have made a very wide variety of firearms and airguns over the past 100 years, so a serial number alone is not enough. Putting up some pictures would allow us tell you about your specific Walther.
Hope that helps, and welcome to the Forum!
PS - Zella-Mehlis is a town in Germany. Walther moved away from there in 1945, when it became part of Communist East Germany.