A small hatch or grill.
{{quote-text|en|year=1860|author=Horace Marryat|title=A Residence in Jutland, the Danish Isles, and Copenhagen|page=51
(small) door, gate (''in wall, fort etc.''); wicket
hatch, grill (in cell etc.)
office, office, booth
counter (at post office, bank etc.)
small door