It means that to an expert on the item in question the appearance of the item is completely consistent with its age. Sort of like people who can make a painting

It means that to an expert on the item in question the appearance of the item is completely consistent with its age. Sort of like people who can make a painting recreating the brush strokes, paints, etc. such that it is difficult, if not impossible, to tell it apart from the original. Now in this case it is a bit different since it is an "original." My Dad never did anything that would make someone looking at the gun think it was some other version, the most common 'ask' there was that Colt Single Action Army (SAA) revolvers, which are famously "The gun that won the west", original generation (i.e. produced from the original Colt dies and tooling by Colt) are more valuable than second, or third generation guns. An expert in Colt firearms can look at an SAA revolver and tell you from which generation it was made with some level of certainty. But lots of guns were abused and stored poorly and developed rust or other imperfections. My Dad was able to cure those imperfections and restore the gun to a condition it would have, had it been stored properly its entire life. And yes, that did increase its value, mostly because the gun just looked better.