Judas Slits or Judas Holes

The peep-holes in a prison-door, through which the guard looks into the cell to see if all is right; when not in use, the holes are covered up.

“It was the faint click made by the cover of the ‘Judasʹ as it falls back into the place over the slit where the eyes have been.”—The Century: Russian Political Prisons, February, 1888, p. 524.