Here are saxophone mouthpiece tip opening charts for modern and vintage saxophone mouthpieces. Please note that the tip openings on mouthpieces vary a lot. So this is a general rule of thumb. This is particularly true of the Brilhart Ebolin and Tonalin mouthpieces. Theo has had 2's that measure like 6's, and 7's that measure like 4's. His guess is it was due to changes in the shape of the facings during the heating and cooling process of putting on the facings. Plastic is much more heat sensitive than other materials. He has noticed this change in hard rubber pieces as well, though to a lesser extent. Metal mouthpieces seem to be the most consistent.