Detail on TL4558 : 4 to 7 Grafton Street. Centuries of whitewashing have obscured whatever light this device might have cast on the original purpose of the buildings
Reality Checkpoint - so called because it supposedly marks the spot, halfway across Parker's Piece, where the bubble of Cambridge University ends and the "real world" of Cambridge city begins. I believe there used to be a plaque inscribed "reality checkpoint" on the lamp-post, but that is long gone and the words are now rather crudely scratched into the paintwork.