As far as I can tell those are the same rings but of different composition (aluminum versus steel).
The difference in Mark 4 rings usually manifests itself in size. There are medium, high, very high Mark 4 rings. Be sure to select a ring size that suits your scope, firearm, and personal desire.
For instance, I had to use high rings on my M1A because I wanted to see my iron sights underneath (in the event I had to transition to them). Had I used Medium rings, I wouldn't have been able to use my iron sights.
The difference in Mark 4 rings usually manifests itself in size. There are medium, high, very high Mark 4 rings. Be sure to select a ring size that suits your scope, firearm, and personal desire.
For instance, I had to use high rings on my M1A because I wanted to see my iron sights underneath (in the event I had to transition to them). Had I used Medium rings, I wouldn't have been able to use my iron sights.