Well done Arp2600, so quick again. These are not easy challenges and I'm always surprised when they are solved so quickly. Yes you are correct of course, Cities that have staged the Summer Olympics from Athens 1896 till the present and the cancelled games for WW1 & 2 you also got. The centuries too. ...... Brilliant. you now receive a 'Gold Challenge Medal', not quite an Olympic Medal but just as deserved :o)
I've spent a few hours thinking about it. I was so sure it had to do with morse code (because dashes and dots and spaces). Those 4 M were quite mysterious. Then it clicked, I was there wondering what list could consist of 30 items, the first beginning with A and the last with R, and suddendly I thought Athens! Rio! ^^