I've had a chance to look at the "newer"? slab side (no bullet design on the side) Bulgarian ak magazines. Using a magnet it seems the feed lips are metal reinforced, has a metal floor plate and the back spine seems to be slightly metal reinforced. I say slightly as the magnet does not stick to that area of the magazine as well as it does to the feed lips. The front spine does not seem to be metal reinforced at all when it was given the magnet test.
If I use that part of the magazine, the back spine, to pick up a magnet, it does so, but not nearly as strongly as the feed lips. The magnet is not picked up by the front spine at all.
I understand the circle 10 mags maybe considered the gold standard as far as polymer ak mags. Metal reinforced feed lips, both locking lugs,front spine and metal floor plate.