I used a broken filter for a Minox 'C' cut down to 7/8" (20 mm). I enlarged the hole to accept the threaded portion of a Series IV filter adapter. The rear threads were cut down so they would not extend through the plastic holder.
A standard Series IV lens hood can be used if desired in place of a retaining ring. It may not be needed, but it looks cool on the camera.
I have a set of standard Series IV filters for color or Black & White, and another set of optical glass filters made by Enteco or Omag which also fit the adapter.
I shot a series of four pictures using the adapter. The film is TMAX 100 developed in Beutler's diluted 1:1:8 for 8 minutes @ 68 degrees. The orange filter received 1 stop increase and the red filter received 2 stops increase in exposure.
Minox Yellow/Green Filter
Exposure +1 stop
Exposure + 2 stops