The difference between the standard and Light 12 is 3 oz. Magnum 12s usually have a hard time cycling 2-3/4" shells, especially target loads. It is a recoil operated shotgun, after all, and the magnum has 2 sets of friction rings and a heavier recoil spring. But I have seen people use them with 2-3/4" shells, they just put the friction rings and one of the brass friction pieces under the recoil spring.
Not sure, but didn't the light 12 have a alloy frame.
The Light 12 still had a steel frame, but the scalloped sides were cut deeper. FN made alloy-framed A5s, but Browning opted to not import them. I picked one up 35 years ago and hunt with it, it is about 6 oz lighter than a Light 12.