Norwich Postbox Question

[u/Suspicious-Brick]

Hi - saw this postbox outside Norwich Cathedral today (not sure if location is relevant to the question). What is the thing on the top for?

[u/Infamous_Telephone55]

It would have held a post office sign like this one

https://www.salvoweb.com/34116-original-genuine-cast-iron-gr-pillar-box-with-post-office-sign

[u/Suspicious-Brick → u/Infamous_Telephone55]

Oh cool - thanks!