Have no idea as to the forend serial number stamp unless it is possibly the last two digits on the entire serial number(?); but all that really matters on a gun in the condition you describe is does the forend correctly fit the frame and barrels? You didn't indicate if this forend was the snap-on type, or the forend type with the push release; but Hunter's snap on forend from this period was a bit weak and were often lost in the field. As your gun has seen lots of use the current forend could be a replacement for that very reason; but again, all that really matters is whether it properly fits.
Having that factory letter is great history and it should be preserved with the gun; just wish you had another stating that it had been returned and what may have been done at the repair shop. But remember if you will that a portion of the gun works building collapsed in February, 1949(the year your letter is dated); and Marlin permanently closed the gun works a few months later in 1950. Maybe it wasn't repaired because your uncle didn't get the gun back before the plant closure? Interesting speculation anyway.
Hunter Arms would always place initials/names on a gun on special request, but the job would have been professionally done; and from your description this work is not professional. Back in the 1960's and 70's gun theft was a huge problem and many gun owners resorted to permanently placing their names, initials, and even SSN numbers on the barrels, frames, and stocks of their firearms. This was often done with an electric tool that punched the metal surface at high speed creating an "engraved" effect (can't remember he name of this tool, but have one somewhere?). This kind of damage can be welded up and the metal smoothed; but since this is a relative's gun, you may want the added character?
I suspect, since your barrels have an odd length, that they have been shortened and were most likely 30" originally. The only history of this gun we can provide would be what is found in the original shipping records; and those records will, among other data, provide an answer as to the original barrel length so I encourage you to acquire a research letter as Drew suggested above.
There are a fair number of qualified gunsmiths who can perform repairs and work miracles on a Smith gun, so I wish you the best; and can assure you that your only limitation on restoration will be the depth of your wallet.