This is a tenor trombone mouthpiece made for King trombone artist Jiggs Whigham. It's similar in size and outer shape to a Denis Wick 12CS (24.50mm cup), but the Whigham has a rounder inner rim edge. For me the Whigham is slightly more comfortable than the Wick due to that different rim contour.
Lightweight outer shape helps give a brilliant centered sound. Rim shape designed by Jiggs Whigham as a super jazz model for upper register work. 24.75mm (0.974") wide cup.