I have just replaced the collar in my Em6910, using (what I assume are) the original instructions. Care and time are mandatory, but the job was a success. The handle now locks positively, zero leakage.
When disassembling, I was somewhat surprised to find that the collar screws were quite loose. I wondered, for a moment, if just tightening them might have saved a replacement. But obviously, as I had bought the collar, I pushed ahead
Anyway, encouraged by the successful restoration, I treated myself to a new M2M stepless grinder. Happy Days!