The Knob Assembly-Winding mated with the retainer-filter to form a single Series 6 filter holder on later models of the shutter the knob assembly winding and retainer filter were merged into a single unit. It also acts as a lens hood.
With the shutter in an uncocked state turn the wind assembly counterclockwise and unscrew it, place a series 6 filter in the recess, reattach by turning clockwise.
On shutters that are in need of cleaning that are continually used, have experienced heavy usage without using a filter, or have been turned hard after reaching the end of cocking travel repeatedly the winding knob will be very hard to unscrew. I have yet to see one that required penetrating oil to loosen but some were close.