Are the version II extension rings backwards compatible with the original extension rings? (n120)? Can you mix and match them? I'm thinking of getting a 25mm extension ring to complement my 20. 30, and 40mm rings to be able to fine tune port length in 5mm increments from 20-95mm. . But I recently purchased the Nauticam Extension Ring 10 Version II and I can't seem to get it to work with any of the other original rings and dome ports I have. Is that an issue unique to the 10mm extension ring? Common to all Version II rings? Or am I just doing it wrong?

They are compatible, there are a couple of changes in them.  I mated a type II ring with an older N85-N120 extension and a 140mm dome.  I wrote up a mini review a little while back as there was no information around about them:

 

 

Thanks Chris!