Here you see the inner workings of a Selmer-U.S. Padless Saxophone.
The Selmer-U.S. Padless Saxophone operates under the same principles as a regular saxophone except that the “pad” is replaced by a flat metal plate and the edge of the tone hole is replaced by a tone ring. To make this work, each tone hole is surrounded by a socket. Soft, resilient "Tonex Tone Rings" are mounted in these sockets. Click on the photo to the left to clearly see the tone rings mounted in each tone hole. These tone rings are composed of three layers of differing synthetic materials that are blended togther to give the saxophone a solid feel when set up properly. The center layer is an air tight, water-resistant layer. Selmer claims to have tested over 300 different materials before settling on the final tone ring composition. A solid metal "Tone Booster" key plate closes on the tone ring in quite the opposite manner of how a traditional saxophone operates. Thus, a seal is made which is effectively the same as a traditional saxophone except here the entire pad cup is essentially one giant resonator. In this sense, the instrument was billed as having “twice” the volume, as being 120% more air tight, as having greater resonance and more brilliant tones, and as having a more defined articulation between the notes.
One major challenge when seting up a Selmer padless saxophone is getting the flat key plate perfectly level to the tone ring. A perfect seal is required. However, once you have the instrument in perfect regulation, it is quite a remarkable instrument. After having taken the time to set one of these up, I know understand the excitement that Selmer put into the launch of their new invention.
Had it not been for WWII, I believe they would have made further improvements on it in terms of overall feel and stability of the set up. Sadly, production was never resumed after WWII as H & H Selmer (USA) began to focus more and more on the student market.