This is an unusual F bass trombone built by Dietrich Kleinehorst in Hamburg Germany. The original owner who comissioned this trombone did not have long arms, and wanted a double valve F bass he could use for symphonic playing.
Note that this isn't a contrabass trombone, but rather an F bass. It's the type of instrument for use on the third trombone part in Romantic music, or fourth trombone in some works. The tessitura of the F bass is similar to a Bb-F bass trombone. (But F is more difficult to play. Because it's F.)
The hand slide of this unique instrument is .547" bore from a Conn 88H, though the connector is King-style. The two rotors are tuned to C, Eb, B. The lowest note with both rotors engaged and the hand slide out is low G. The low Gb above the fundamental F can be played if you pull the first rotor slide out a half step ("Pull to B"). I believe you can play the Barktok B-F glissando by using the second rotor and releasing it partway through.
Two bells are included:
A custom hard case is included, along with a large Verner Schmidt mouthpiece, though most any bass trombone mouthpiece will fit. The second rotor paddle is removable to simplify storage in the case.
This Gronitz is a weighty instrument. It may be fatiguing to hold due to the wide bell section and lots of tubing. The weight adds some torque back and to the left (back and to the left). I don't easily envision a solution with some sort of hand rest, though the Kuhnl & Hoyer hand rest may be a good start. The fit is complicated by the removable bell connector bracket. Your mileage may vary.
Hand slide has moderate lacquer wear but excellent action, and overall the instrument is in very lightly used condition, ready for your next orchestra set.