Fin the opening in an open polysurface?

Ho do I find the opening in an “open polysurface?” This happens often when I’m working with screw threads. I never create them myself and often the imported threaded object turns out to be open polysurface. Cap sometimes fixed it by not always. Usually, I just trash it and find a better model. At any rate, here we are again.

I looked under analyze, but could not fins an obvious tool to highlight the opening.
Open.3dm (906.8 KB)

I use ZoomNaked and then the commandline option All to show openings. You can fix this perticular model by running RemoveAllNakedMicroEdges.

Thanks. That was not where I expected to find the problem! I did something for that to happen.