The "Schilke - Helleberg II-F" has the same rim as the Helleberg SH-II but with a shallower bowl-shaped cup to accommodate the smaller F tuba. The throat is smaller than the SH. The backbore is slightly larger, similar to a Schmitt shape that opens up quicker under the throat to reduce resistance. - the manufacturer California Residents Only: WARNING: Cancer and Reproductive Harm - www.p65warnings.ca.gov