Now I have know Dan Higgins for many years.
When he came to me about making some strange instruments for the "Cat In The Hat" movie I said why not.
We started by cutting the neck off an alto saxophone.
Then we took to bell off and put a small baritone horn bell on it. I couldn't make a noise by Dan played a lot of notes. It turned out the new alto was almost a B alto with a bunch of funny notes.
Next: We took a metal bass clarinet I had and put a French Horn bell and a C Melody neck together. Well this was strange to me but Dan again had fun playing new....... and I guess we can call it an instrument....... and of course Dan played it in the movie score.

This tenor has an additional high Eb key. Placed level with the High Palm E key and on the front side of the saxophone. Making some repeated passages much easier.
The key on this instrument only worked the high Eb.

This is the harmonic or altissimo key on this old Holton works and looks much like the YAMAHA harmonic key.

Under the harmonic key you can see the extra tone hole for the alternate high note trill key.

On the original Holton (The Carl fischer name appears on this instrument) had a key in the same position as this one. The Holton key however was over an additional tone hole. This key with the extra tone hole would allow the saxophonist to go from high C to D by opening the key. They could go from high C# to D, D to D#, D# to E all with this key. It made many passages seem like a piece of cake to play.

You will notice several interesting keys on the right hand of this saxophone. There is a bis type key for the second finger of the right hand allowing the third or D finger to work just the top key. this gives a D to Eb trill. There are three keys for the right hand little finger key. The third key gives a right hand C#.

The modern register key is full of adjustment problems. There is always lost motion and if it is taken completely out even the smallest change in the pad can make it not work. However the original double register key meant you would need to change register keys between G# and A. Playing a middle D to an A .... change the register key. I feel it is easier to live with a little lost motion in the modern single register key.