That means that if you already have the GOG or Steam version installed, you can download any Retail version of Mount & Blade: Warband from ftp//ftp.taleworldscom and install it to a different folder on your computer. If you have the GOG version of Mount & Blade: Warband, it should work like the Steam version and write your serial key to the registry. I do not have the GOG version, so do not ask. I may be able to modify it to work with that version. Test it and let me know if it doesn't work with the version you want to use it with.
I know some people like to use previous versions for Mod compatibility. Now updated for v1.168 (Should work with some other versions.