The filter holder was made by Leitz. It replaces the red filter and you need a special longer screw with it. Not easy to find. You can also use filter holders of other brands, with a small modification.
I think it was made in the US or in Canada, when poly contrast filters were introduced. If I am correct it is listed in one of the catalogs of Leitz US. Obviously it was (almost) a service to owners of the first version Focomat 1C without the drawer for colour filters. I think these were not made by Leitz in Germany, but I am not sure. It is just that they never show up in Europe.