The tuning mechanism - is the inverted "U" shaped item, between the crook, of the horn, and the valves.
There is another item that was optional, and that's the Bb to A changing mechanism. I think it was something of a novelty - just an interesting looking gadget, that seemed pretty neat, until the musician realized he was going to have to remove it, or at least part of it every time, in order to to clean his horn. It's such a tricky thing to do, most horn players just didn't put it back on, after they took it off. It looks like an arm that runs through the crook of the second valve slide. When the main tuning slide is pulled out, it sets the arm in motion, which, in turn, pulls the slides, so that they are all automatically in the position, in which they need be, in order for the player to play in the key of A.
My 1921 Victor has this arm. I've circled it in gold:
21 Victor [IMG][/IMG]
My 1917 Victor doesn't have the arm, as you can see, but I believe the player can manually adjust all the slides in order to play in the key of A. (Someone please correct me if I'm wrong.)
To be honest Albert, horn players change models all the time. Sometimes they just want something different, (or just can't turn down the opportunity to buy a shiny, new horn to try) - different models all have their pros and cons. The Conn Victor, depending on the mouthpiece used, can change its sound from that of a cornet, to the mellower flugelhorn, or bright like a trumpet. It's very versatile, but the downside is that the bore size is very large, and can be very tiring to play, over a long period of time. It really takes some work, to build the physical strength to play that horn. I'm sure Andy could tell you all about it, after all the playing he did around the Bix Tribute.
My impression, as to why there are those who think Bix didn't use the Bach much, was that
A. The Bach, #620, looked relatively unused.
B. There are no pictures of Bix with either Bach.
It would be interesting to know how much work the horn needed in its restoration, if it actually ever was restored. Mantler could've told Mary Louise that it needed restoration work, in order to gain possession of it, for all I know.