Sorry to be so slow, but I have distractions. The triangle for maj7 is a product of the MENC, Music Educators National Conference. The quality of its ideas is, to say the least, uneven. The second time I saw it was on a chord sheet at a jam session. We all agreed that it was a bad idea. I can't think of anything worse than inventing a symbol which looks like an o, an a, a D, or even a natural sign when it is written in a hurry. Since at least 1969 I have used a backslash through a 7 or 9 to indicate a maj7 in arranging tunes like "We've Only Just Begun" which have enough maj7 chords to cause a linear space problem. It is easy to use, totally intuitive, *does not require new symbols* in a logical sense, and unambiguous. The idea was inspired by the slash through the o for the m7b5 or "half" diminished 7th, a great shortcut, which I understood and appreciated instantly the first time I saw it, in an arrangement by the late John Harrison for Charlie Byrd. Don't you wish you'd thought of that? Or did you? :-)
