Tombstone Gabby
My best guess is someone mated a 39-A barrel and left receiver with a Model 1897 right receiver.
The mis-match in the two halves of the receivers seems to indicate this, as does the later 39-A features and markings on the left side, and the early Model 1897 marks and features of the right side.
The barrel has been drilled and tapped for a old Marlin barrel scope mount and the receiver top have been drilled and tapped for a scope.
However the receiver side has not been drilled and tapped for a receiver sight.
This may or may not have been a factory job.
This leads me to suspect that the 39-A half is an early version, either pre-war or early post-war
I'm not sure what the brass looking surface is on the lower tang. The tang is one piece with the receiver and is steel. A look inside with the stock off might show the tang to have been brazed on after being broken off, or ????? could just be discolored bluing
The Model 39-A versions would have the serial number stamped on either the top tang or the bottom tang. The Model 39-A should also have the serial number stamped inside the left receiver half below the bolt, so you can figure out when the right side and barrel was made.
The early Marlin serial numbers were confusing and I can't find a resource that tells when an 1897 with serial number 4344XX was made.
In any event the right side of the rifle was a Model 1897 made sometime before 1922.
The left side was a Model 39-A made sometime after 1939. Again, if you disassemble the rifle and look on the inside of the left half, there should be a serial number that should allow figuring out when that half was made.
You still need to figure out what era of bolt is in the rifle. Again, on rifles made before the mid-1930's, you should shoot ONLY Standard Velocity ammo or risk a broken bolt.
Here's a site that shows how to ID the bolt.
http://www.wisnersinc.com/rifles/marlin/rflever.htm
My suggestion is to get on rimfirecentral.com and ask on the Marlin forum for serial number help. Some people there should be able to date the two halves and give you more info.
As for value, probably not a lot due to the mis-match and the poor condition of the stock. The mis-fit of the stock indicates it's been sanded down or replaced. Look inside the stock tang areas for a serial number.
As for "How's this happen", this is at least the third or fourth Marlin I've seen over the years either as a mis-match or as just one half of the rifle.
These were easy to disassemble and often got broken down for storage as the two pieces.
Things happen and one half disappears.