Nikon F 4 Instruction Manual page 97

Nikon Eyepiece Magnifier DG-2
Provides 2x magnification of the central portion of the finder
image with Eyepiece Adapter DK-1. Eyesight adjustment pro-
vided. Useful for critical focusing in close-up photography.
Nikon Eyepiece Adapter DK-7
Lets you attach the DR-3 or DG-2 to the Multi-Meter Finder
DP-20's eyepiece.