Tea Kettle register tube.
The Tea Kettle register key design as it is known was very brilliant. It raised the entire register tube to the outside of the saxophone neck allowing more air to go through the register pip (Register Tube). It also had a split register hold at the top of the pip splitting the air as it passed through and helped clear up many of the strange notes of the saxophone. On the tenor it really helped the A above the staff a generally weak note. You can do the same by putting a very small, thin piece of brass in the center of the register hole. I have seen this on both Selmer altos & tenors… Great design!