This is a LETO 506 violin, viola, and cello rosin. Rosin is a solid form of resin from pine trees used on instrument bows to create friction with the strings, which produces sound.
It is a dark formulation, which tends to be softer and provides more grip and a brighter tone.
The formula originates from Austria.
It is suitable for use on violin, viola, and cello bows.
It is typically applied to the bow hair every few hours of playing time.