They have. I corrected my last to clarify that I have both the mag 20 and a 'Light Twenty'. The one variation I don't have is the mag 12.
The 20 mag will cycle any standard 20 gauge load. The Browning brochure from 1967 was included with my gun and that was the whole sales idea with the mag 20.
There is an internet article by Wakeman to the contrary, but my gun works perfectly with a standard skeet load. I shot 75 more today.
There are two friction pieces and associated rings. For the light loads, use one friction assembly on the muzzle side of the recoil spring and store the other between the spring and the receiver. For heavier loads, stack both friction pieces on the muzzle end of the spring.
Both the 20's are good handling guns, but the mag 20 really is just like a Sweet 16.
If you find a mag 20 Ted, it's worth playing with. It's really a hoot.