This 1688 is a rare large bore tenor trombone made by Kanstul in Anaheim, California. It's similar in shape and style to the classic Conn 88H trombone, but the Kanstul's twist is locating the tuning mechanism in the hand slide rather than in the bell like most trombones offered today.
Like a trumpet, a trombone's bell section is designed to have a conical taper. Locating the tuning slide in the bell section, though convenient, interrupts the proper taper with the addition of the tuning slide sleeves. Moving that mechanism into the hand slide allows a lighter, properly tapered bell section, and the tuning adjustment is part of the cylindrical portion of the instrument. When adjusting the tuning length, a tuning-in-slide trombone has has no affect on the tapers, and simply adds a bit of length to the outside slide tubes. To look at it another way, to tune a 1688 flatter, you're really moving the hand slide brace closer to you. It's simple, it's lightweight, and the benefits of tone and response are quite amazing. The repsponse is lighter, the low range is more open, the tone is move colorful - it is a magical feeling when playing one of these horns.
Why don't you see more? Most builders cater to the student market, and younger players may not have the discipline to properly care for the tuning mechanism on this horn. And...most people don't care. There are some very fine bell tuning trombones made, so only a few players have asked for a horn like the 1688.
The 1688 has fairly standard specs: .547" bronze slide with nickel trim, 3 removable mouthpipes, a oversized Kanstul CR rotary valve, and the bell section is made of all lightweight bronze. A heavier gauge red brass bell is available by special request. The red brass gives more defined slotting up high and more projection, with the expense of some extra weight in the bell flair.
A Protec 306 hard case and mouthpiece are included.
The last photo above shows a 1688 next to the non-F-attatchment version called the 1608. A slightly heavier balance weight (recommended) can be had on the 1608 by adding an additional cross brace across the bell section and using a standard weight.
Here is a nice review from Eric Burger with thoughtful compliments and criticism. For 2015, Kanstul has improved their buffing and polishing, lacquer, and cleaning facilities. We think their new instruments should be their best finished ever.
Here's Eric talking about the benefits of trombones that tune in the hand slide like the 1688.