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Dave in Maine:
I was trying to be as simple and straight forward as possible so he could disassemble the gun and figured if I so much as mentioned "water table" and "standing breach" etc. the owner would have decided we were all nuts and went somewhere else.

Jim
Someone upthread had mentioned "watertable" and I figured it would be a good idea to define that while I was at it.
To the owner: we've been mentioning "chopper lump tubeset". To clarify, there are several ways of making the barrels for double-barrelled guns, one of which is called "chopper lump". To keep it short and sweet, this process involves making the two individual barrels each from a single piece of steel. Each of these pieces of steel will have extra metal at the butt end of the piece, from which the manufacturer will later form the "lumps". The "lumps" are those blocky hooks of metal projecting from the middle-bottom of the barrel flats which lock into the bottom of the receiver. The individual barrel blanks are machined to shape (and bored) and the lumps on each are machined to shape - half of the lumps on each barrel. The two barrels are then joined together (usually by brazing) at the lumps and other work is done to join the barrels into a single unit. The name "chopper" comes from British English. The raw barrel blanks with the lumps look something like an axe, which is called a "chopper" in British. Thus the name.
Making double-barrelled guns of any caliber by the chopper lump method is time-consuming, requires a lot of fine craftsmanship and detail work and is therefore reserved for expensive, higher-end guns. Is it "better" than any other method? You'd have to define "better". It is surely more expensive and involves more fine craftsmanship.
And, FWIW, the "standing breech" is the flat face on the receiver with the holes for the firing pins and against which the back end of the barrels rests. It's perpendicular to the watertable.
The 9.3x74 cartridge is one which is still in production. It is quite powerful and used in big game rifles. IIRC, it is legal for elephant and cape buffalo in some countries. I would not, however, go out and buy a box to shoot through this gun. At least not just yet. (Setting aside that it'll cost $4 to $5 each time you pull the trigger and your local gun store is unlikely to carry it.) This gun needs to be carefully inspected by a gunsmith who knows about double rifles, detail cleaned, and the bores checked carefully. This gun will turn 100 later this year and deserves to be treated with care. It also needs a trigger guard.
Since it hasn't been fired in many years (no surprise - the ammunition is obscure) I would not be surprised if the hammer springs are shot and will have to be replaced. Being left cocked for 70 years will do that to even the best steel springs.
In the meantime, keep it in a warm, dry place. I'm sure someone here will be able to suggest a good gunsmith to do what's needed to be done.
Congratulations! You hit the lottery....