There are a lot of subtle differences in these filters. I didn't actually look it up, but there are many intricate little features that could be tweaked. For instance, I have a 304 and the darn thing was REALLY hard to get going at first. My water level is so low, and the filter is on the floor. There is a little handle you're supposed to pump to get the siphon going, but it didn't work at all for me. The newer filter may be improved in that area. Also, there are several compartments for filter media, hose connections, gaskets, and even basic style that could be different. I LOVE THIS FILTER!! It is so easy to maintain and keeps the water crystal clear. The starting thing is no longer a problem since the system is full of water now. Just make sure your turtles can handle some current. These filters are pretty powerful.