JohnP wrote:

Out walking and idly looking at the various manhole covers and hydrant
covers.

I saw one that had this cast on "Sewer Wash Out".

Purpose obvious - but why and were are this situated? It was fire hrdrant
sized.

Fire appliances carry a database of all fire hydrants and washout
hydrants which can be used as second preference source of water

see
https://www.ukfrs.com/sites/default/files/2017-09/Fire%20Service%20Manual_Volume%201%20-%20Fire%20Service%20Technology%20Equipment%20and%2 0Media%20-%20Hydraulics_%20Pumps%20and%20Water%20Supplies.pd f