“A number of devices for simplified writing are current among children and young people.” [Not much has changed in 80 years, has it?] “A rhyme popular among young people reads:

YYUR – Too wise you are,

YYUB – Too wise you be,

ICUR – I see you are

YY4Me – Too wise for me.

“Musical expression is also made use of for abbreviated writing in popular lore, e.g. ‘Just a little ♪ to you.’ Some well-known mottos for musicians run B▀, B#, but don’t B¶ [yeah, I know that’s not a flat sign, but it will have to do] (be square, be sharp, but don’t be flat) or ‘always B#, never B¶, always B≡’ (always be sharp, never be flat, always be natural) [yeah, that’s not a natural sign either, but you get the idea].”

Apparently these were already quite common in 1924.