The rubber isolator has a brass ring on the inside with a bayonet fitting. Unless someone has glued it on, you should be able to twist gently anti-clockwise (when looking from the front of the soundbox)until it stops, then gently pull it off.
The picture below (from the Nipperhead site) is for a #2 soundbox, but the concept is the same.
I hope this helps.
Best regards ... AZ*