The one on my viper (similar if not the same - it's been a while since I owned a proper Mamba) I could unscrew with the cap closed. You may have to pull it a little while unscrewing to add slight pressure to the threads.

When it's loose, open the cap and, grab the inner and outer ring with your thumb and finger at the same time, then unscrew until it comes off.

Failing that, like Grumpy says, knock up something stable to fit the 2 holes in the inner ring.