I'm a little lost tbh. Is it like the later the letter, the more recent the release? Are they separate games or something? 
Should I just play the 2L version to experience everything or do I need to play through all the letters?

Hi frostymm,

Yes, play version "L" to access the latest content (at the time I'm writing this). You can ignore the older ones. We add a new build every 3 weeks or so, and the older ones are left up if someone wants to download them for whatever reason. The letters are just game build designations.

The newest version of the game is always the latest upload, and has the biggest file size.

Okay cool, thanks for clearing that up!