If the file contains JSON, it should be named .json not .txt.
Another thing i would like to do with the code is rather than show the markers with letters i would prefer them numbered.
Somewhat surprisingly, and most definitely aggravatingly, the Google Maps API makes it fairly difficult to put text on a map. You have the option of using a single character for markers using
Marker Labels but if you need more than that, you need to create a custom overlay that creates custom HTML elements to put text onto the map.
I think I have such an overlay laying around that I could share if you like, but it's not simple.
You'd think that such a common and simple need would be provided for, but no-o-o-o-o-o-o-o.
