You know, I don't really know anything about this British wheel other than that it's 7.5 inches across. I'm guessing it dates from World War 1. It's in excellent shape. I hope its owner never needed to refer to it.
Its a serious and scary-looking wheel, isn't it? If you are exposed to whatever D.M. stands for, you need to watch out for loss of self-control. For B.B.C., combat fear and panic by reassurance.