fred, I open the file in meshmixer as you suggested and did a plane cut. Here are 2 screen shots, one with no fill and one with a mesh fill to highlight the construction. I did not create the stl, but it sure looks like construction artifacts.
First no fill:
Plane-Cut-NoFill.JPG

now, with mesh fill.
Plane-Cut-RemeshedFill.JPG

The stl is on thingiverse here -> https://www.thingiverse.com/download:3896657

So even if they are construction artifacts, what can be done in S3D to make a good print?