The small size frayed brushes are called "ashirai kebo" in Japanese.  They are used to pick up keshi-type matte gold or silver powder and sprinkled onto lacquered surface (after "ua-nuri"stage).  As with all our kintsugi material, these are made with natural hair from squirrels.
 
INFOMATION 1:  S-size (ashirai) is usually used to pick up gold/silver powder. M-size (harai) is to brush off excess powder from the surface of the repaired ceramics.  
 
INFORMATION 2:  since these brushes are not intended to be washed, at minimum get one for gold-tone powder and one for silver-tone powder.  This is to prevent contamination of color.  E.g. You may not want your silver effect to have gold-tone in it.   Or you may not want your precious pure gold to have residual of copper/bronze or other color powder.   Best to use a separate set for different metallic materials.