Trimble R8 model 3 lemo connector tool
I just mic'ed the one sitting on my desk and I got a shy 16mm or 5/8". A 5/8" wrench with a flat nose if you can find one. You could also grind away on a cheap 5/8" 6-point socket until you remove the 1/8" or so from 4 of the faces. Mine is on my desk because the Bluetooth doesn't work as well, good luck to you!