Triangle (Delta) chord symbol

[dflat]

Every time I see the triangle with a 7 I have a little pang of annoyance

Me too.

I am very curious as to when and how the 7 was added to the triangle ?
Was it started here at iRealb?

... (to answer your original question) no it was not us that started ^7 as an alternative to ^ by itself. However is has been convenient for us since we do not have room for 'maj'.

Personally I too learnt and prefer ^ without the 7 when meaning maj7. I wonder whether after some musicians started using ^9 that then the ^ was misinterpreted as meaning maj, so the 7 was added. Another point, ^7, ^9 and ^13 in the same chart make it clear.

We do offer both options so if you are annoyed, you can always change it (it annoys me too but I cannot be bothered changing them—sigh so I am used to it now).
Using ^9 (or ^13) might be specific for some reason, for instance the melody being the 9th or another instrument would be playing that note (big band) so the guitarist or pianist would be given this specific chord quality so they can voice it appropriately (and it gives them more voicings to choose from).

[marcosfilho]

Is ir possible to write C7M instead C^ ?

[Bobsax]

Is ir possible to write C7M instead C^ ?

No
Why would you want to?
C = Major

[marcosfilho]

Here in Brazil we use 7M to mean seventh major, like Maj7 in USA. So, C7M = C, E, G, B. Here in Brazil we also use parentheses to separate the different changes, such as Cm7(b5) or C7M(#9), similar to Jazz Session app. I thought iReal was more flexible with other types of chord symbols. The fact that the player does not recognize is no problem for me. Thanks.

[dflat]

Is ir possible to write C7M instead C^ ?

Do you mean would the player know what it is? If you put it in, it would probably show an error when it goes to play it. You could write in C^ in the measure so the player knows what to do, and add C7M either as an alt chord or text.
Obviously if you are not using the player it will input, but might not show in the chart fonts we use.