I hate to answer for Giz since he's so knowledgeable about these things, but "mounting and furniture" refers to how the gun is set up and the things like the barrel wedges, trigger guard, sights, tang, cap on the stock, ramrod and thimbles, buttplate, etc. Those things are generically referred to as "furniture."

I hope Giz checks back in and corrects if any of this is wrong, but I think that about covers it.

It's a very nice gun. I really like the browned barrels. I hope you plan to shoot her!