Looking for a reference of when = was officially accepted (I guess that would be by the ITU-R) as the symbol for -...- ?
I think it may be older than that.
The Deutsch-Oesterreichischen Telegraphenverein was a Union of German and
Austrian telegraphers, that formed in 1850. They met in convention in 1854, where
they adopted a convention on numbers, punctuation, and operating signals.
The resulting code was published in the US by T. P. Shaffner, in 'The Telegraph
Manual', published in 1859.
The code is described in chapter 34, which you can download and read here:http://www.unitedstatesmilitarytelegraph.org/files/telegraphmanual/ChapterThirtyFour.pdf
The 'Austro-Germanic Morse Alphabet' is on page 4 of the pdf.
Notice that one 'dit' was been dropped from the code for '=' at some point.
It started out as