IBM 5280 User's Setup Procedures page 25

4. Remove and unwrap the two mirrors from the top of your
data station. If the mirrors are smudged, clean the mirrors
with glass cleaner.
Mirrors
5. Insert a mirror on each side of the display screen as follows:
Lay the mirror on the top or bottom ledge of the mirror
housing; then lower the bottom of the mirror into its position.
User's Setup Procedures 19