What you have is the fore-runner to the Vickers MG slide rule. The brass disc is held to the eye and the line is stretched to arms length and you sight through the vee. The later slide rules had moving scales that could be used for direct comparision, whereas your one would have to be judged off the scales stamped on the brass. A very nice item that would be right at home in my MG collection, should you wish to dispose of it! The slide rules were introduced to replace the old range tables, but were complicated to use. The Vickers dial sight replaced them to a large degree. I have seen no reference to your range plate in any book, but it is obvious what it is when compared to the later slide rules which were first introduced about 1923. Your range plate was probably made during WW1, but not an official issue item.



























