Looks like they all fit together. On the CDNN website they have a stock for the Winchester 1300/1400 and it says FN 12 gauge right on it, but only on the one. I called them this morning and although the guy wasn't overly convincing at all he said they all work fine. And a few people over on Shotgunworld.com also confirmed this, so it looks like it should be all good.
Now to decide which stock to get. I like some of the Speedfeed ones so we'll see.