This iris arrived in the post with no sender information, so no provenance. After considerable research I think it MIGHT be Hyperion, but am open to alternative suggestions.
Trosupurba x Dominion.
TB-M-B3D (AIS Checklist 1939)
Lightest blue-toned flower of the Dominion race. Standards bluish lavender. Falls clear bright pleroma violet. Strong grower. 3'6". Light blue toned flower of fine but rather long form. Very large flower with long arching standards of bluish lavender 2 5/8" broad. Falls 2⅛" broad smooth and hanging with tilted tips, bright pleroma violet. Branching, free flowering 3' 2"(AJB 1920/21 + 1921/22)
Light blue toned flower of the Dominion race with finely formed flowers. A really magnificent grower and free flowering. The individual blooms are very large with long arching Standards 2 5/8" broad, bright bluish lavender. Falls are smooth and hanging with tilted tips, over 2⅛" broad, a clear bright pleroma violet. Nearly 3⅛' high, with strong branching spikes. A fine iris. (Whitelegg Orpington 1923)