The backbore is conical and narrow. Egger, A. - provides an A1 or an AB mouthpiece. Both have a flat rim, or, if desired a round or rounded rim. Custom made mouthpieces are also available with screw rims. 22mm Doug Elliott makes a model 2 backbore for the alto with an 's' shank.
Various rims are avbailable.
Layhill rd., Silver Spring, Maryland 20906. Glassl - provides a variety of mouthpieces for the alto sackbut. Cup depth is medium shallow. Cup depth is medium shallow. Cup depth is medium shallow.