How fabulous that you have letters from you mother's mother. What language are they written in?The café was often referred to as the "Chess school" (Die Schachhochschule) because of the presence of many chess players who used the first floor for their games.
Wow - I'd love to visit there as well. My grandfather played chess with Trotsky in Switzerland between the wars - or so I was told - but maybe they first hung out in Vienna.
Vienna is a lovely city, I visited last week but didn't have time to visit the Cafe Central. I'm definitely going to revisit it. The letters were in German, although they were later translated into English by one of my relatives. Unfortunately I only have the English copies. I would love to try translating them myself.