Apologies I should change the topic to "pros and cons of Al-Mg alloys vs carbon" topic.
RM clarified that the carbon nano tube case (or case back) replacement is only available for early generation / prototype / initial batches of RM035 which had some issues with black coating fading away and revealing the metal. Reference to "case no longer being available" is for the 91% Magnesium AZ91 cases. Newer batches use 95% Aluminium alloy and Miarox coating which seems to have solved the issue.