The markings were generally used on all aircraft. And of course, depending on the aircraft and engine type, the markings varied.
However...I have found both early and late instruments panels that did not have any markings at all...it was really dependant on the ground crew in many cases.
The "M" codes on the He111 panel are electrical codes, so the mechanics and ground crew know which plugs and connections go where and for what purpose.
mike