The International Orchid Register can be accessed on the Royal Horticultural Society's website (
here) for just such questions.
Curiously, they list two separate entires for Slc. Anzac! There's one registered in 1915 as Sc. Bletchleyensis x Sc. Clive Lawrence, and another one registered in 1921 as Sc. Marathon x Sc. Dominiana, as littlefrog pointed out.
Any idea why there would be two kinds of crosses carrying this epithet?