what I can see on You pictures is, that the part copy seems to be a mesh body.
The question now is, which SE version are You using.
The usability of mesh bodies (STL, etc.) is much better with ST10 than in previous versions.
The part appears to be only composed of surfaces. If you show surfaces in the assembly you should see. You could probably fairly easily make it solid as well.
Or it's what @hawcad suggests.
Hi there @KurtCouffez,
By default, SE will place [& display] only "Solid Bodies" into an assembly, and there is an option to show construction geometry, if no solid is present.......but in your case, I think you'd need to just use the [RMB] context menu for "Show/Hide All Components" to display all else.
@hawcadthat doesn't appear to be a mesh body. When you translate an stl the thing that shows up in the design tree looks like this
Horrifying thing, stl files.
Oh, I suppose. Once. However it is so large that it won't let me post here. So try this, I uploaded to my drive. Hope this works.
Hope the link works---->Alien