Terry,
From the shape of the action, I would guess Stevens as well. I've got an old 12 ga. gun that I'm about to scrap in order to experiment with some new welding rod. I would be glad to send it to you, as quite often these forends fit well enough from one gun to another, but I'm not certain whether or not they changed frame size from 12 to 20. My initial thought would be that they should have, but these were inexpensive mass produced guns, so who knows. Try Numrich's and see if they list different part numbers for the 20 vs. the 12's. If not, your welcome to the forend that I have. It is tennite, however, not wood.
Luck,
Jim
Oh, it appears you'll need a trigger guard as well. If this old thing I've got still has one, you're welcome to it too.