Not a super sharp image. I managed to knock out the filler that was at the ends the fret-slots. with my makeshift .020 serrated feeler gauge, which also worked pretty well for cleaning out the slot.

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I have now cut some anigre veneer to go in the slots. Why did I choose anigre, you wonder? Because it was (a) light colored and (b) the only veneer I have. I got a 12" x 29" piece of this stuff about two years ago and have been using it slowly to correct mistakes every since. The internet tells me that it should have about the same density as maple, so it should be OK for this. Got to be at least as good as wood filler ;-)