The JM mark varied through the years as the die(s) were changed.
The initials were for John Marlin.
The mark was applied in the Range after Proof (1rd), Function (5rds) and Targeting (3rds) of the firearm.
Orig the mark was hand stamped with a single cut die stamp and hammer.
Late 60's the switch to a die stamp inserted into a small hand held pneumatic hammergun made punching the cart full (30 on a cart) of guns a quick operation after they came from the Targeting shooters (3 rds for Targeting.
One Range guy ran a somewhat oily patch down and back out of each bore quickly as another banged the Proof mark onto each gun as they sat in the cart.
Since the above was always done on completely finished and assembled firearms, the mark should be visible above the wood line.
It's always a possibility a gun was missed getting a Proof mark. It was a Factory atmosphere just like any other. Hectic, loud, keep things moving..
Guns right after that visit to the Range were wheeled off to the packing and shipping dept.
The JM Proof mark was also supposed to be a mark of final inspection in the Range. No one looked over each gun individually as far as I saw.
A glaring fault got caught, maybe,, but small things,,not a chance.
Setting those a side meant that the guy in the range was dumping on the Final Assembler's daily rate. You didn't do that.
Same as the Final Assembler didn't nitpick on Sub Assemblys for his/her work done up by one of the Sub-Assemblers.
Piece Work,,,We all just want to get along
I don't know that many/any were pulled from the line over at Packing for defects seen in fit or finish, but it's certainly possible.
I doubt many did though.
When things got hectic and the Range got behind,, racks of guns were wheeled in, proof marked and wheeled right back out again to Ship&Packing, Never seeing a Proof, Function or Targeting round.
Get'er Done...American Mfg'g at it's Best.
The Proofmark should be punched Thru the Blue (or Plating),,not underneath it if original.
That's true for any firearms mfg'r.
Parts & Service Dept mailorder replacement bbls may have proof mark on the under side of the bbl or an off side flat if an octagon.
Seems like they had a slightly different Proof mark than the standard JM on an Oval,,maybe not. Winchester used a different proof mark for their replacement bbls sent outside the factory.
I think Marlin stopped the practice of mail-order replacement bbl quite some time ago.
Most all of any older replacement parts Marlin had as of the mid 70's they sold off to Numrich.
Numrich sold many of them directly to other dealers/collectors, so they never saw the general market. The rest started to show up in their regular Shotgun News and other ads but not till the early 80's IIRC.
They had a warehouse full of stuff, but the word given to the general public after about 1972 when ever someone would send an older classic MArlin or LCS in was 'We have no parts available' and they'd send it back.
