I have a 7.62x39 Romanian SAR-1. I ordered 4 smooth side Arsenal mags and 4 Bulgarian waffle mags for it. I had to do some hand fitting on all of them. I used a dremel and a sanding wheel. Yes, there is metal under the polymer in places. On a few of the mags I had to get remove a little of the metal. They all work great in the gun now. I have always had 5 East German steel mags as well. One of these has always stuck in the magwell and been hard to remove. I hit it with the dremel too, now it is smooth and easy to remove.
Guns like AKs, especially parts kits guns, have a wide range of tolerances. My experience with many things AK is that hand fitting is in order - be it mags, grips, buttstocks, hand guards, etc. I imagine alot of magazine makers make their mags on the large size. That way they fit the guns with big magwells, and can be ground down to fit in guns with small wells, or the wells of the guns enlarged. If the mags were made too small, you wouldn't be able to correct the problem. Always easier to take some material away than add it.
I chose to take material away from the mags. I didn't want bare metal exposed to corresion on my gun. Also, if I went too far and screwed up, it was a cheap magazine, not my rifle getting messed up. Although some other posters have brought up good points about enlarging the gun's mag well so that you know any AK mag will fit.