When I bought my Leica MS5 had been modified with flat caps where the knobs should be.  Note this does not connect like a normal Leica knob, which would take a bolt through the end.  I am not sure if the MS5 is different from zooming models or if this is a result of the modifications done to this copy, but there was no internal thread to accept a bolt.  Instead these knobs attach by sliding onto the larger round horizontal shaft inside the microscope and then accepts an M4 bolt through the side.  M4 threads will not fit more than ~1.5mm into the existing hole in this shaft, so if your M4 bolt is too long you will have to either shorten it or grind the threads down so they will fit into the hole.  The left side of the shaft was also very slightly smaller than the right side, so I had to slightly shim the left knob with a bit of plastic packing tape for a tight wobble-free fit.