Mentioned only by Wedgwood in his entry for Solo, in which he says: A prefix signifying that a stop to which it is attached is intended for Solo use; e.g., Solo Diapason. The prefix sometimes also serves to indicate a high pressure flute stop of powerful tone.

Solo Diapason 8', Bombarde; St. George's Hall, Liverpool, England; Willis 1855. This is the only known example.