Williams describes these stops as �16' or 32' manual trumpets in the treble compass only (i.e. the 32' was as an 8' trumpet beginning at c1), probably always en chamade in those exceptional instruments to contain them.� Grove says: �Trompeta magna ... and Trompeta imperial were horizontal Trumpets, often of suboctave pitches (16', even 32', in the treble).� Imperial has the same meaning in English; magna means �great�.
Osiris contains dozens of examples of Trompeta Magna, nearly all at 16' pitch, a few at 8' pitch, and many en chamade, but only two examples of Trompeta Imperial.