BTW Marlin is an old company spanning generations and there have been many variations over the years. When inquiring about an ID the more info the better. Examples:
1) Does it have a saddle ring or evidence that one existed?
2) Barrel length?
3) Approximate age?
4) Stock shape?
5) Checkering?
6) JM or JM in an oval?
7) Cross-bolt safety?
8) Cartridge capacity?
It also helps if someone more knowledgeable than me responds.
JM in an oval is the barrels proof mark. I can;t remember the JM without the oval as most of mine are post WWII.
Others have covered most of the after letters, A few more are GT (Glenfield texan), SS, (Stainless steel) DC (deluxe carbine), SRC (Saddle ring carbine), LTD (Limited edition) ltd II, LTD III LTD IV, CB (Cowboy) and a few more strange ones. Ever just the plain old 336 with no other markings. Then there were the sears, ted williams, monkey wards, western auto, big five and glenfild brands that marlin made over the years.
Also marlins at times in the past would put overrun barrels on weird platforms. A barrel with RC stamped with straight grip stock ect. Plus since most of the parts are mix and match often peoplle would themselves make one working gun from 2 or more old rifles and come up with a FRANKENMARLIN.
Steer clear of anyting post Remington. There is some interesting information on what happend upon the remington aquisition that took them a while to catch on to and then I believe Remington was paying by way of "piece" rather than hourly. Can someone speak to that please.
Check to see which rifling it has:
Micro-groove: After 1955, lots of shallow grooves, some claim it is better for jacketed bullets.
Ballard: Fewer and deeper grooves, some claim it is better for non-jacketed lead bullets.
If you can, cycle a few rounds through it to check for jamming issues.