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If you can't find a replacement it is sometimes possible to sand down any 'knarling' and re-blue with decent results.
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I find it better to use a smooth-faced hammer and tap the ridges back down. If you do it carefully, the screw will look almost undamaged after a touch up to the slot with a thin file, but there will be a finish loss. I then try to duplicate the original metal polish texture on the screw and follow with cold blue. I hate a "buggered" screw, and this method eliminates that ugly sight on an otherwise nice gun.