An octave Tromba, at 4' in the manuals or 8' in the pedal, voiced somewhat less prominently than the unison Tromba which usually accompanies it. The name Octave Tromba is mentioned only by Bonavia-Hunt; we assume it to be a synonym for Tromba Clarion.
Osiris contains fifteen examples of Octave Tromba, and six examples of Tromba Clarion, all at 4' pitch.