I'm stuck. I searched for all local copies of the lighthouse01.nif and deleted them, so it should only reference the built-in one. I deleted any mods that affected the lighthouse; fully deleted; no just uninstalled. However, when I use the TCL console command ingame, I can plainly see that the material is still two sided. I created a material swap, gave it a name, and applied it to the base item. You can see it becomes one-sided in the CK. I don't know how to proceed and I think I have to give up. If someone can point me to mesh editing, like what tools do you use, I can maybe try that route.