Did you mean "Clacks"? Or was it an attempt to point to https://fr.wikipedia.org/wiki/Sudac_air_service which has no English version AFAIK?

https://parisianfields.com/2018/03/25/a-question-of-time/

Fascinating but mislabeled. This is about the Paris Pneumatic Post, or underground tube message system.

Video reference: https://www.youtube.com/watch?v=gol_p2aWrJg