The JM is not a guaranteed indicator the entire gun was made at the old factory.
Two years ago when going through the racks of display leverguns at the Marlin booth at the SHOT Show in Vegas I was told by two different Marlin people that they continued using up existing stocks of "old factory" barrels on new guns built at the "new factory" till they were gone.
What he said ^^
The "REP" and "JM" stamps are not 'proof marks', as many people seem to think.
It is simply a manufacturer's stamp for the barrel.
REP barrels were seen on some North Haven Marlins, before the move; and JM barrels were seen on some Ilion/Mayfield Marlins, after the move.
The transition from JM to REP occurred at what ever point in time the stock of JM barrels ran out. For some models, it was well before
North Haven was shut down.
The serial number (year of manufacture) is far more useful than just avoiding "REP" barrels.
If you ask around, especially on the Marlin forums, you'll find that most people can't tell the difference in performance between the JM and REP barrels. Some people have even argued that the REP barrels are superior
Regardless, that stamp only tells you who made the barrel. The rest of the parts, and its place of assembly, are more important if you want to avoid "Remlins"/"Marlingtons".