Hi guys - sorry, been very busy. Now have a few days off, but will be going to visit my folks for Thanksgiving, so will probably be off the board again.
Orpheus - yes, as I believe Rok said, those are mellophones. By the way, the term Stan Kenton uses in that video is incorrect, he calls them something weird like mellophoniums. Anyway, they are basically large trumpets that have a mellower sound, closer to my instrument in tone quality. Larger than the fluglehorn, which you may be more familiar with, as that is used much more in jazz than the mellophones were, and the sound of a flugle horn is closer to the trumpet sound than the mellophone is. In fact, I think Stan Kenton's band was the only one that used mellophones on anything like a regular basis. They are most commonly used in marching bands. The trumpet players in that video were playing them, however in many marching bands, an extension is put into the lead pipe of the instrument so that a French horn mouthpiece can fit, and then horn players play them. They are truly awful instruments, with no redeeming qualities whatsoever. In recent years, an instrument called the marching French horn has been invented to replace mellophones, so they are not used much at all anymore. They were quite common, though, nothing unusual about them, in fact I think it is unusual that you haven't ever seen one - you probably have in marching bands without realizing it.
In the two years I spent in a high school marching band before I escaped to an arts academy, I had to play the mellophone. Miserable experience. I have played some Stan Kenton charts that had mellophone parts in pops concerts, though we played them on our regular horns. They were screaming high, so were not very pleasant to play. Decent enough charts. That was a very popular band, guys like Maynard Ferguson and other famous trumpet players came out of it. Nothing particularly interesting about it from a purely musical standpoint, frankly, but they did have some good tunes.