JM stands for John Marlin, the old timer who built Marlin in the late 1800s. The JM stamping is a clear delineation point for buyers to know if a marlin rifle was made before or after the Remington buy out of family owned Marlin in 2009/2010. A JM stamp means it was made before the takeover.
Remington screwed up quality big time in the beginning of the takeover, Google it to know more. So JM marlins are coveted for quality, craftsmanship, and having no surprises like some of the REP stamped rifles(Remington Peters). But not all Remington Marlins have quality issues, its just a hit or miss type thing that rightfully is a concern for buyers purchasing a non-JM gun sight unseen.
Thanks fer the info! I've never been a lever type guy, always a sucker fer bolts! I always wanted a 35 REM so I got this one and will keep the JM in mind if I ever see a good deal on another....
OP, good luck on your search.