The Type K
The Type K combines a split-image center with a surrounding microprism collar. Align the two halves of a straight edge to focus, or watch the collar sharpen. It is a familiar all-around choice for manual-focus shooting.
The Type J
The Type J uses a central microprism rather than a split-image line. It offers a less divided view in the middle of the frame, which some photographers prefer for subjects without strong straight edges.
Check the screen itself
Inspect the focusing surface for marks and confirm the retaining frame is sound. Scratches on a screen may show in the finder even though they do not appear on film. Verify that the screen is the F3 version; a similar-looking screen for another Nikon body is not automatically interchangeable.