Not a lot. Folks tend to forget that people once actually shot these things! Sure, it is nice to have everything original, but this is a valid period replacement to enhance accuracy and is part of its history. I wouldn't replace it and I wouldn't deduct much from the value. If it really bothered someone, a "correct" sight is an easy replacement. Some would consider replacement "boosting"...I consider it "repair". Boy, am I going to be roasted for that comment.
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If it's made after 1918...it's a reproduction
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