If it is on a shooter, and it matches well, I do not think more than a 10% discount is needed. If it was on a very high grade or a very collectible gun then 30% seems reasonable. In fact I have bought several shooters that were mismatched and if the price was decent to begin with I did not press the issue.
I think that we obsess on this "original condition" and all "exactly factory original" too much. These are guns were made 75 plus years ago and were owned by people who put more store in the effective use than their potential to gain value. Maybe they have or had the right idea.
