'M' vs. 'MJ' metric thread profile?
The "J" suffix designates that the threads have a controlled root radius to enhance the fatigue properties by lowering the Kt factor (stress intensity). Look in Mil-Hdbk-28 for a complete description of the this thread form. The "J" form is popular on aerospace bolts which are also thread rolled after heat treatment to retain the compressive stresses in the root. Unfortunately aerospace and metric are mutally exclusive.... European airplane makers use the English system for their fasteners. Good luck.