Sorry, it's been waaaay too long since I've been on the Nautilus
(back in the 70's). I can't really remember the interior of the ship other than there were two rows of seats that ran lenghtwise facing the portholes on the side of the ship. I don't remember there being anything fancy with the seats, other than they folded up/down for un/loading - and I don't remember any fancy ornimentation on or around the seats themselves. I do remember that there were stairs to get in/out of the ship, but I can't remember what kind of ornimentation they had.
I'm fairly sure that your part came from the interior portion of the ship - just wish that I could help you out a little more. I've posted a copy of some exterior drawings of the Nautilus, but I don't think that they'll be of any help:
Nautilus In any case good luck, and please let us know if you find the answer.