I actually owned a clone, can't remember the name now but made in SA, at the time I thought it was the last word in modern comfort.
Having carried a modern pack for the last 35 years all I can say is "rugged and dependable" and if I was a collector I'd have it in a shot.Carrying one of these is probably part of the reason I have a bad back now LOL
There were many versions of the MM, that picture is of the Light Weight, it substituted nylon webbing for the thick chrome tanned leather on the straps and shoulder harness.
There was a "tanker" version that had a hollow frame with a tap on the bottom for carrying stove fuel and a "Hunter" version that had a horizontal shelf at the bottom rear to allow you to carry drums , jerry-cans and deer carcasses more easily. Huge capacity and when fully packed was basically a great big cube sitting hard on your back and shoulders, but there was an extension sewed in to allow packing even more in and could go well above your head, big side flaps were then tied tight to hold it all together.