M = manufacturer = Motor Wheel Corp. (a subsidiary of GOODYEAR Tire & Rubber) 5 = plant #5 = Chatham, Ontario, Canada (opened in '66, is it still there?)
There were two of the same nuts like it is said above, one with no dimple the other has one. The dimple is what you want to be OEM. Use to have bags full of both threads, used to put them on everything that took a factory steel wheel.
