MakerBot Print is our newest print-prepration software, which supports native CAD files and STL assemblies,
allows you to interact with all your printers via the Cloud, and many other exciting new features.
Here you can see the difference:
right knurled: https://cdn.thingiverse.com/renders/ae/c4/48/b3/1f/Lid50_right_knurled_preview_featured.jpg
left knurled: https://cdn.thingiverse.com/renders/ba/29/2c/a3/d1/Lid50_left_knurled_preview_featured.jpg
The threads are are always right handed, but you simply mirror the model if to get a left handed jar.
If you want to get rid of the warnings, simply replace assign(...) expression with let(...). But let(...) is not implemented in the OpenSCAD 2015.03 used by Customizer. So we have to wait until Customizer is updated.
To check current Customizer version:https://www.thingiverse.com/thing:640677