In a way, this is an extension of the Barrel Lengthening thread.

I can recall reading a number of times about an original muzzle loader that had been handed down in a family.  Over the course of some 200 years, the barrel had been 'freshed out' a couple of times, internal lock parts had been replace (especially the frizzen) and it had to be restocked (or a piece added on) some 100 years ago.  With the provenance, this is touted as an original muzzle loader.  But that is a muzzle loader.  Let's look at single shots.

1) The barrel has been bored out and re-rifled.

2) The barrel has been bored out and a liner installed allowing the rifle to be re-chambered to the original cartridge.

3) The barrel has been cut off retaining the original barrel markings and another barrel is turned down to fit within the barrel stub and re-chambered to original chamber.

4) The metal has been refinished and is indetectable.

5) Internal parts (especially the extractor) have been replaced due to wear or breakage.   

6) Wood has been replaced (using antique wood with original finishes) and been distressed.

7) All parts used are either original factory parts or salvaged from original rifles.

8) All the work was done at the factory over 100 years ago.

9) All the work was done by a custom gunmaker.

10) The entire rifle was made by an accumulation of original parts... lock, stock and barrel.

Where is the line?  At what point is it necessary to mention repairs to an old rifle?  At what point is someone being cheated?