What an unexpected treatment of a wholly anticipated theme, and congratulations to Shackleton for thinking of it.
As for "prothesis", Greek had such a word, but it was first introduced into English in the 16th century, via post-classical Latin, in a grammatical usage, meaning the addition of a letter or syllable, usually at the beginning of a word. In the 19th century it came to mean an antecedent entity or purpose, in a religious or spiritual sense. For more, see the OED, and particularly the 1831-2 quote from Coleridge.
According to Hegelian dialectic one has, not a prothesis, but a thesis and an antithesis, the contradiction between which is resolved by synthesis. That then becomes the new thesis, and the contradiction between it and its antithesis is resolved by a new synthesis. The process is then repeated until Olympian perfection ensues.
So, strictly speaking, what Shackleton has called the antithesis is actually the thesis, to which Set 2 (his own invention) is the antithesis, the final grid being the synthesis. Not far off perfection.
Incidentally, this is also something of an anniversary, since the missing vowels were first designed in this form in 1912.