The reason I am fairly confident it is a JM stamp is that I see part of the oval and the beginning of the letters adjacent to the oval so it is pretty clear to me, having seen similar in my search for the perfect Marlin over the last 50 years give or take, that the stamp is a pretty poor example of an off square effort at a JM stamp. And it is in the place that I see most, at least of newer pre-Remington Marlins, where the stamps are often found. The oval leg gives it away, to me. Could be wrong, no expert here for certain. I can clearly see the REP stamp on my two Remington Marlins as it is machine made. On my JMs, not so much.
If the rifle is nice and you like it I would not keep that from allowing me to purchase it. But like I said, for about two years before to about two or even three after circa 2010, the transition period, I would scrutinize it very, very carefully.
3C