Whoever told you it was an R13 was full of poo. It is an R10 Darne, definitely post second big war. If you looked at the gourmet hunter website, the gun in the take down pictures gun is an R13.  I've posted enough times, right here, how to get a Darne apart, either an R or a V, so, I'll spare everyone that. Search right here, if you get stuck, PM me and we will get it down for you via electrons, I guess.
This isn't rocket science.
If it fits, you are lucky indeed, noted above. If the stock is hollowed, you might have some issues getting a Silvers on the gun. I always wonder about a 20 gauge Darne with a pad, they shoot so sweet with just a buttplate, using regular loads, and the guns are dainty enough that putting a block of rubber on the back sometimes changes the balance the wrong way.
Not for me, thank you. If I needed a pad for length, I'd use a slip on.
 None of the advice you have received here today is bad, Dr. Sane pointed out the obvious for you, but, he does that. There was a time when guys told you to get out a mallet and some big screwdrivers, and maybe a vise grips to get a Darne apart, maybe this whole internet thing is maturing a bit, no?
Use it in good health. Questions right here, if you got 'em.
Best,
Ted
 
Ted:
Thanks for the information.  Yes, the stock is hollow.  Some owner before me glued an extension on the stock, which hasn't been cut, however.  The guy I got it from didn't know from anything about Darne (nor do I, but I'm learning.)  He thought it was a 13 because of the shape of the action.  I cancelled the order for the Silvers pad and instead found a repro buttplate, which will make the gun about 3/8" short, but there it is.
The stock needs refinishing, which I'll do eventually.  The finish now isn't awful, no dents or anything, just a rather dull and ordinary finish. The wood isn't interesting at all.
I saw one of these guns when I was in Viet Nam, (former French colony) about 1969.  I can't remember exactly in what context I came across it.  But I didn't know what it was then.  I do remember it had a bright receiver, I think.
I finally got the barrels off the gun with a little help.  The barrels were really tight and took some persuasion (without hard tools) to move them back. Wonderfully simple once you see how it's done