Transitional Style H (and possibly Style J)
Solo Valve System

(Photograph courtesy of Don Teach)

The rare transitional solo system was used only briefly in 1917, judging by the only known example (#74,785). It uses a different valve system with pipe chest pneumatics that are reversed from normal. All the pneumatics must be collapsed to keep their notes from playing, and letting air into a pneumatic allows a small hinge spring to open it and play the note. The valve system remains undocumented at this time.