In the first edition of the sheet music, the metronome marking for Etude op. 10 no 2 is 144 beats per minute – and the piece is in common time (4/4 time signature):
However, in one of Chopin’s manuscripts, he wrote the metronome marking as 69 BPM and indicated cut time, so 2/2 time signature:
Because there is cut time in the manuscript, this tempo would be 69*2=138 if notated in common time, as in the first edition. Perhaps Chopin changed 138 to 144 for the official publication, so that it looks more ‘decent’ – but this is only my personal speculation, and I’m completely not sure of that.
It’s worth pointing out that Chopin was likely aware of the ‘special’ meaning of the number 69, since he was born in 1810 and the term originated at least as early as 1790s. According to Wikipedia:
The term sixty-nine or soixante-neuf for [CENSORED] is an English translation of the euphemistic French term, “soixante-neuf.” The term “soixante-neuf” has not been traced any earlier than the Whore’s Catechisms published in the 1790s in France (…)The Wikipedia Community