So this was news to me, but perhaps I'm just slow on the uptake. I posted this photo to Twitter earlier today

Take a look at the three long indicators on the 30-minute counter- at the 3, 6 and 9 minute marks. These appear on quite a few of the '50s chronographs...not just from Heuer



What are these three markers for? At this time telephone call charges were calculated by way of a pulse every 3 minutes, therefore allowing the wearer to monitor the length of their telephone call. Heuer understood its male customer base too, as no man talks on the phone for more than 10 minutes!

Thanks to @jonpwatches on Twitter for the info.