For a time I owned a 1941 Universal Geneve Bi-Compax chronograph. It was a fantastic watch and was made of the most amazing red gold, however something about it baffled me. On my watch, and I soon noticed on others from the same time, there were elogated lines in the minute totalizer at three minutes, six minutes and nine minutes. It was so specific and done across nearly every brand, from Longines to Wittnaur to Rolex to Breitling, I knew there had to be an explanation.
I turned to the great minds in the Reddit watch community and was told a very interesting answer, which I thought needed to be shared.
It all comes down to the telephone. According to a watchmaker and enthusiast, I was informed that back in the 40s, 50s and early 60s when these watches were being produced, people used payphones regurarly. Cell phones obviously didn't exist and many people didn't have landline in their home yet. When using a payphone at the time, the money you put in got you three minutes of talk time, and you were cut off abruptly when your time was up.
The lines on the chronograph simply help you keep track of your telephone call. You'd start the chronograph, put in your money, and easily be able to know when to put more money in or to finish your conversation. Most calls were likely under 10 minutes, which is why only the first three-minute markers look like this.
And for your enjoyment, a gallery of vintage chronographs with the lines. You'll see that some companies put them at 4, 8 and 12.