Galle records an old Ghent Hybrid, ‘Cuprea Grandiflora’, orange coloured, blotched with orange-yellow which may be this plant.  The 1836 edition of Loddiges’ Nursery catalogue lists Azalea hybridae-belgicae cuprea with six sub-varieties, alba, elegans, eximia, globosa, rubra and splendens.  This perhaps makes it more likely that Macarthur’s Azalea cuprea is a variety of this old Ghent hybrid.  Such plants are now collectively called Rhododendron [Azalea] x gandavensis Hort.