This is a small bore tenor trombone designed by Steve Shires and made at the S.E. Shires factory in (its former location of) Hopedale, Massachusetts, USA. I've had this for some time; it was built when Steve Shires was with the company and spun the bells.
Steve's original design for a small bore trombone was based on the classic model 6 by Earl Williams. A favorite of west coast players, the Williams had a large throat 8" bell like a King 3B, but had a smaller .500" bore hand slide for efficiency. The Williams also had an extended and gracefully curving tuning slide bow. Shires Custom Series small bore trombones, no matter the model, are based on this Williams design with its distinctive bell and tuning slide shape.