Please visit this website:

www.msv-woelbling.at/download.php?fileID=40

Thats a PDF file with the homologations of mufflers of EFRA. And you can see, the 2601 is a bit shorter which means it will provide less bottom end compared to the 2602. Theres also a few tiny differences. As you can see in Murnan's post, the word "almost" are these tiny differences. As I mentioned before, the 2620 and the 2602 are very very similar to each other, only small difference is the bigger outside diameter of the 2620 by about 0.5mm.

Hope this helps.