Suffix depends on the specific manufacturer.
MJE340G would be the current generation On-Semiconductor parts. "G" = "Green" or lead free with a TO-225 body style.
On-Semi inherited the Fairchild Semi MJE340S components for a TO-126 body style.
Both TO-126 and TO-225 are very similar in body style.
Differences:
MJE340S parts have three dots on front to indicate front of transistor. Standard pinout - Left to right = Base, Collector, Emitter.
MJE340G parts have a heat slug to indicate back side of transistor. Same pinout.
Sometimes you will see a "TU" at the end of the Fairchild parts (e.g MJE340STU). This simply means the parts are shipped "Tube" packaged.
"K" suffix typically means it is a TO-3 transistor. But the MJE340 does not use a TO-3 style case so the parts list is using some old suffix from a no doubt defunct manufacturer.