What do you mean? I've bought 3 extra mags for mine and they are from 446C version. There are slight differences in new mags but regarding converting them back to 18 - they are the same as the old ones - impossible to convert unless you either cut the plastic insert or get the magazine floorplate from the 18-rounds mag.
At Baikal they show me custom papers in red ink explanation, and if u do have plastic plug in it - be aware it's not legal. That's what i been told