I've uploaded the STL... I have checked it in multiple apps and it seems to be interpreted correctly.
I don't understand what you mean by "not a top" surface... I have included a screenshot from Cura 15 which gets the shell correct. While the 6 itself is not a top surface, the hole in the 6 IS a top surface. In fact, Cura knows it is because, if I enable ironing, it tries to iron that part but there is no shell to iron! I've included a sceengrab of the ironing layer with ironing enabled.
I am indeed slicing for a non-UM printer. Does that matter? I a