One of the things I sort of obsessively look for and photograph in churches are elevation squints. These little openings can often be seen cut into the dado of medieval screens and were created to allow the faithful kneeling in front of the screen a glimpse of the Elevation of the Host taking place at the altar beyond.
There are loads of them about and they are of all shapes and sizes. Sometime they are crudely cut like arrowslits in a castle wall, sometimes care has been taken to make them blend in stylistically with the screenwork. They are for the most part amateur affairs and were very probably cut by the faithful themselves.
Lawrence Lew OP, has a wonderful view through the squint at Cassington:
Martin Beek has captured an elevation squint in use at Stanton Harcourt, Oxfordshire: