Which file should I choose?
Use SB115215_3D-simplified_2D-front.dxf (revision 1.0) to insert the supplied editable drawing into your layout.
Use SB115215_3D-simplified.stp (revision 1.0) to place the original solid geometry in your 3D assembly. The ZIP contains the unmodified SB115215_3D-simplified.stp.
