Your barrels and for-end iron date it to 1909. I believe that the beavertail wood was added to it later. The receiver's serial number dates it to post 1945, and could be from parts left over when Marlin bought them out in same year. If that is all that is stamped on the receiver's water table, but, I don't believe at that time they were making the Regular frame then. Is there a number stamp like 00 and then E on the receiver's water table? If so this would date it to 1906
The above date quote is incorrect; your frame dates to 1908 and the serial number block denotes that this gun originally shipped with factory optional auto-ejectors. The barrels/foreiron serial numbers indicate 1909 prodcution and is from a serial number block indicating that these parts were originally on a gun shipped with manual extractors. As Smith gun ejector parts are all housed within the forearm and barrels, barrels with manual extractors of the same frame size/type can easily be fitted to what was formerly an ejector gun frame; while on the other hand, it takes a lot of work to convert a set of manual extractor barrels to function with auto ejectors.
We know that your frame is from 1908, as it is an "R" (Regular) sized gun frame; and "R" frames had been dropped from production by 1945. Other clear age/period indicators are the shape of the grip and checkering pattern. Why does your gun not have the original barrels? We will never know of course; they could have been damaged, but it is also very likely that your gun was originally a Grade O gun which would have had Damascus barrels that were replaced with steel tubes at some point.
As to the loose short rib, this could be the result of several causes. I suspect your barrels have been reblued at some point; if these barrels were improperly hot blued, the bluing salts used could have reacted with/dissolved some of the original rib solder and weakened the bond. The only way to know for sure is to have the barrels inspected by a qualified doublegun smith. Should that be the case, it is possible that all ribs should be relayed and then the barrels properly slow rust blued. Another possibility is that the replacement foreiron was not properly fitted to the frame and placed too much tension on the hanger lug; eventually breaking the rib loose during recoil. Again this must be determined by a gunsmith, and is a problem that can also be repaired.
As to the large hole in the short rib, this is not factory origial. I have seen small holes aftermarket drilled in this area to insure hot bluing salts were removed after a reblue; but in most cases a large hole in this area indicates that someone did a little creative gunsmithing and added a screw or bolt to secure the forearm after a broken "J" spring. In your case I don't see a corresponding hole in the iron itself, so can't say for sny certainty; but there is some sort of non-original screw attachment added to the front portion of the iron. Again, have it checked by a gunsmith.
As to repair, with the barrels in their present damaged state and the altered iron and aftermarket beavertail; you certainly don't have $400 worth of parts. Were it me at this point, I suppose I'd have the barrels repaired and the forend returned to the original splinter type (Smith beavertail options were not available on guns from the 1908/09 era); then I'd shoot and enjoy the gun. Properly repaired and maintained this old Smith is good for another 100 years; good luck.