[CQ-Contest] origin of "es" in Morse code (cw)?

[Tim Shoppa]

There are written and printed forms of the Ampersand that can look a lot
like "es".

The Ampersand itself is not an abbreviation of "es". It is a ligature for
"et", the latin word for and.

There are stylized forms of "ET" ligature where the "T" has curlicues on
its top which if laid over a little look like a backwards S. The most
extreme may be Baskerville Italic:
https://indesignskills.com/tutorials/fonts-with-the-best-ampersands/

Tim N3QE