Peter
Ok my post was abstract, I apologise, I tried to thread the Leitz filters onto the front of the 3rd party filters and none would thread, without undue force by more then one turn after start. The filters should thread easily.
So I tried one third party on the lux and yes it did thread without any force. It was this that worried me the most. I have other lenses with damaged threads and they can be a pain.
So I quarantined all the 3rd party filters without seeing if they would foul with the hood, some looked as though they might.
I use the 43mm with a 40.5 to 43 mm step anyway so there is no real loss, the Leitz uv, green and orange are pretty much all I use on a fast handling M, I'd prefer a light yellow, but too expensive today.
I did not want to risk a cross thread some time in future with the lux...
Even more worrying for me was the label on the 46mm showcase, that it was 0.75 pitch.
Again I apologise, and I had not detected any problems although I may never have used the 3rd party filters ever.
Noel