Yeah, I was feeling kinda agitated about it all yesterday and got my Grrr up. Now I'm better, and thanks for being able to point that out to me.
I didn't just buy the wrong product, as I called Tim at Nordic Components and we discussed this very thing. I told him what I had, exactly, and he said that this particular Mag Extension would fit my SuperNova. I asked him if he was sure it would fit, he asked me how long was my barrel, I told him 18.5", and he said that he had to ask in order to make sure I was getting the correct length, and that Yes, the +3 is exactly what I needed. So I felt assured that it was going to "fit".
What I didn't take into account was our definitions of "fit" were different. I meant "would it attach and be flush with the end of the barrel", whereas he evidently was concerned with the thread of the barrel nut matching up with the thread on the Mag tube.
Like you said, live and learn.