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 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