This is a hand rest used to support the weight of the trombone on the back of your hand. Similar hand rests are available from Hagman, Kuhnl & Hoyer, Haag and others. This one was designed for the Kanstul 1690 contrabass trombone, but it can be used with other instruments. Simply order the hand rest, and then also order 2 $8 shims in the diameter of your instrument's main brace.
Made of stainless steel and brass, with foam padding similar to a mouse pad. Lacquered.