Knowing the work was done by the factory is definitely makes the gun more valuable than wondering if the work was done by some random gunsmith. I don't think the work makes the gun worth much if any more than factory original. Maybe $50 more at best due to its uniqueness.

Values vary $50 to $100 all the time, every day. Due to the condition that gun is worth some where around $400 to $500. Maybe it'd go for $600 at an auction with a couple bidders fighting against one another, or maybe it go for $350 at an auction with poor turn out. What I'm getting at is that trying pin an extract value of something like that is difficult and highly dependent on the venue where it's being sold and how many interested parties are there. I've seen the exact same gun go for more than a $100 difference just depending on when a gun is sold in an auction.