JM stands for john marlin. the barrel right near the receiver has a small JM stamped on, denoting a gun that was made in the new haven factory before remington moved production to ilion ny. after the move, marlins have a REM stamped on the barrel near the receiver. the JM's are more desirable than the REM's. the REM's had unbelievable bad quality for several years and there are a lot of them out there. buyer beware. that said, the newer REM's appear to be better quality-wise, but remington has cheapened the whole line with laminated stocks with cheesy checkering, and dull matte finish instead of blue. if i was looking for a 39, i'd look to 50-60's production guns. to determine the date of mfg, subtract the first two digits of the s/n from 2000 and thats the year. i.e., 24xxx would be made in 1976. this varies a bit on older guns and early 70's guns but google can help you figure that out.
