According to Ricci Adams, it's M:
Major seventh chords are abbreviated with a capital 'M' and a 7.
Sibelius (version 5) uses MA.
The wikipedia page "Popular_music_symbols#Chord_quality" says the following:
M or maj (and Δ is sometimes used for major), where M is the short version and maj is the long version.
So the possibilities look like this:
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