No one has asked yet, but, if you don't mind our asking, what is the serial number.  The carbine looks interesting and, if it is in an appropriate number range, it is still quite valuable as an 1898 Carbine.  As Whig said, there are 1896 Carbines out there with rifle sights, they are nevertheless, real 1896 Carbines and worth more than the rifle versions.  The incorrect sight would be a deduction from the value, but hardly a disqualifier for the rest of the piece. Even with an inappropriate number, carbine parts are always worth something more and, unless the whole thing is a fake or a "put together" like the one I bought when I was a novice, the sum of the parts will come to a happy result.