If you do get your hands on the Opticron mm3/4 50 HD and can compare it to your Nikon 50ED, I‘ll be very much looking forward to your findings!I think the Opticron 50s were not available when I last looked at them, but happy to see them back in the pipeline. I suspect these are tougher. I like their clamshell design; seems strong. I have not looked through one, and will be ordering to compare and will report back. If it's as good or better than the ED50, I'll keep it and sell the ED50.
(I would also love to compare them, but that doesn’t seem possible here in Belgium.)