The DK-3 rubber eyecup has a metal ring embedded at the front for the eyepiece to grip as it clamps the eyecup to the viewfinder.
The bottom of the eyecup is pushed to flex it upward with a finger to clear the back as you open or close the film door.
Itís important to orient the eyecup with the cutout facing the film door (facing down). You can see the cutout facing downward in this photo.
To orient the eyecup properly requires that you test it. Youíll see that the friction of fully tightening the eyepiece grabs the eycup and forces it to rotate the eyecup clockwise about the same angle ɵ each time.
By noting this angle and initially positioning the eyecup ɵį counterclockwise as the eyepiece begins to forcefully grip the eyepiece, youíll end up with a fully tightened eyepiece with the eyecup rotated into proper position with the clearance cutout at the bottom. This makes it easy to flex the eyepiece upward and out of the way for the film door.
Itís much easier to do than to describe.