Most double French horn models can be categorised into these two design layouts...
Geyer horns are named after their German born American designer Carl Geyer. You can recogonise them by the layout of their rotor valves.
The Bb rotor - controlled by the thumb key to turn the horn from F to B-flat - is located in line with the three other valves. This configuration results in less bends in the tubing, giving the F side of the horn less resistance and allowing a better blended sound between F and Bb sides of the instrument. This also helps the horn achieve nicer slurs between notes.
The John Packer JP261 Rath and JP261D Rath French horns are Geyer wraps.
A maker of brass instruments in the German town of Eisenach, Edward Kruspe is believed to have built the first double horn prototype.
The Kruspe design layout features the Bb change valve close to the thumb key. This configuration results in several tubing bends that don’t exist in the Geyer design, leading to the “closed wrap” name. There are some advantages to this extra tubing, including more options to tune the horn on the Bb side and also easier ability to empty water from the instrument. These bends also result in more resistance, which the Kruspe design counterbalances with a larger bell. This resistance combined with a big open bell, gives the horn its unique playing qualities and characteristic dark tone.
The John Packer JP164 French horn is a Kruspe wrap.