It is a "proof mark" used by Marlin to show that the barrel was installed on the receiver by Marlin, and met quality standards.
On older rifles, it is an actual proof mark, and did indicate successful test firing. But, later on, it turned into nothing more than a "maker's mark" and QC stamp.
When Remington moved production to Ilion, NY, and Mayfield, KY (and I think I'm forgetting one location?), they went to Remington's stamp of "REP". ...which coincided with massive quality control problems.
So, the JM rifles are far more desirable than REP rifles - mainly because the odds of getting a basket case are MUCH higher with REP on the barrel.
There are some oddities out there, as well, that have both the JM and the REP stamps. These were take-off barrels that got put into stock after the North Haven plant shut down. When Remington started production, they installed some of the QC-failed JM barrels on production rifles, and then over-stamped the JM with REP, or stamped the barrel with REP in addition to JM. Those are the most trouble-prone rifles of all. Avoid them like the plague.