Music Theory: what's the correct notation for writing a major 7th, 11th, 13th? M vs MA (or Ma) vs maj vs Δ

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:
CM7
CMA7
Cmaj7
CΔ7