I have replacement Truss Rod Nuts available. This style nut was used from 1966 to 1976. They changed to a round nut with an internal 5mm hex from 1977 to 1980. In 1981 the truss rod adjustment was moved inside the guitar. The Nut will come with a small washer that goes behind the nut.
See my forum for the most current info about Replacement Parts for Vintage Yamaha FG’s.
The original Truss Rod nut is on the left. It is rather ugly because I think they finished the guitar after it has been assembled, coloring the exposed surfaces brown with mahogany stain.
The new truss rod nut is made from low carbon steel, with the same hardness as the original. It has a dull nickel plated finish. The internal threads are metric, but not a current standard thread.