While lemon oiling the fretboard my rag snagged on the face of the nut; the rag's fuzz is still there in these pics. Close inspection with a magnifying glass showed tiny seating burrs in the bottom face of the string slots where the strings vibrations migrated into the aluminum nut. I will remove the nut and apply a tiny spot of silver-tin solder to the floor of any string slot that needs it and then reface those slots with small diamond files. It doesn't look like much in the pics (nor to the eye) but the floor of the strings slots slope downward towards the tuners, so every little bit of nut meat vibrated out of the slot floor by the strings, means that the strings sit lower by that amount and likely too close to the first fret for optimum playing geometry.
