Although the CAD drawings are shown in a static position Adrian, on the computer the axle can move through its full arc and thus I think your concerns have been allowed for.
I've long thought that a set up of this sort is "the next step".
There is nothing wrong with a well sorted rigid rear end, IRS is better but far more complex and expensiveness and in many respects would spoil the character of the MOG. I'll bet that with this rear and a Suplex front the Roadster rides and handles remarkably well!
Although I had the Suplex suspension and the Rutherford AVO's fitted to my Roadster, the 'mule' was running standard Morgan front suspension as regards springs, and with the rear end secure the front also seemed to be less of an issue.
As regards the provenance of the rear axle, I'm sure someone mentioned that it was manufactured in Malaysia, though at whose OE instigation I don't know. Maybe it is off a baccy, next time I'm in the factory I'll ask them Paul?
