The way I mdae that part wasn't with extrude... the whole thing is a large gun, made originally as a .obj file. Then I took another .obj file of a small open top box, stuck it on the side, and exported the whole thing as another .obj file, then converted to a prop. So the small box that seems to be the crux of the problem does indeed have a surface beneath it. But I didn't think that would be an issue since the box top was open.
SAMS3d, I'm not sure what you meant when you said
"Try in UV inverting just that one section, save it and look at it again in Poser."

Here's the link to my freebies:   https://www.renderosity.com/mod/freestuff/?uid=493127

My cousin Jack can speak to beans. That's right.... Jack and the beans talk