+1 on Colin's comments. Basically, the kit nuts are generally rubbish. The bone nut upgrade is worth the small price, however the upgrade is not available for all kits. Definitely worth sourcing a bone nut or the TUSQ mentioned above if you can't get a suitable version from PB. I don't know the specs of the JM-1 so I don't know if a standard F-style nut will just slot straight in there and it seems the only bone nuts on PB that are specifically for lefties is for a bass.
It's never too late to learn to play right-handed. :-)