In the 1920s, nightclubs in Berlin utilized a unique system to facilitate late-night flirtation between strangers. The Resi and the Femina were two nightclubs that pioneered this trend. At the Resi, table phones and pneumatic tubes allowed for anonymous communication between patrons. The system was a major draw of the nightclub, with phones fixed to individual tables and a lighted number above each one. The process involved looking for an attractive stranger, noting their table number, and sending them a message. The Femina also had a similar system with pneumatic tubes built into handrails. These pneumatic tubes were used to send notes, much like messaging on a dating app. However, messages sent through the tubes were checked by female “censors” in a form of comment moderation. The system existed for decades and was remembered fondly by Americans who visited Berlin after World War II. Fictional accounts and novels have also paid tribute to this unique method of communication.
https://www.atlasobscura.com/articles/pneumatic-tube-table-phone-flirting-berlin