WARNING... These comments are pure conjecture, and demonstrate the ignorance of the poster...
The ancient makers were making a tool to be carried into the woods everyday. They were making art at times, yes, but mostly they were making a tool. I think we, as modern recreationist, forget that. They were not concerned about a gun surviving hundreds of years, but a mere five would have pleased them. We too often assign our current minds to those of 250 years ago, and it simply does not work.
Frankly, I am amazed that the guns have survived with no special care all of these years. That being said, most people in the 1700s did know how to take care of wood. It is not unlikely that men would have rubbed linseed oil into their guns after every hunt. This would have protected would under inlays, and thus, the old builders didn't bother with finishing areas that could expand, and prevent reinsertion of inlays and the like. I believe there are several accounts of men using animal grease in the barrel channels, which would be a definite anti-rot effort. I find no reference to this being used under inlays.