Yeah there shouldn't be any nuts on the back side. I helped a friend pull his off on his Moomba to seal them because he said they would take in water. Just unscrewed them. One screw hole was stripped a little and it just spun like you mention, but we just put outward pressure on it and it came right out.
Now...that's not to say that they may have done things a little different at the shop. When I had a Moomba, I was all the time finding inconsistencies throughout the boat, i.e. different nut/bolt sizes where they should be uniform, different wire connectors, etc. But then again it was a Moomba which is the cheaper version of a Supra...most likely they were put together using whatever was left over from building the Supras! lol
Last edited by beg4wake; 02-03-2016 at 6:38 AM.