Anybody knows what this is ?

[Quasimodo]

Hello,

While on bunkertour in Denmark in 2007 we were asked to find out what these "things" are.
An old couple have asked me what the object on the picture was.
They live in an old (refurbished)German barak, and they found in the ground 21 of this things. (The barrack was part of a coastal artillery battery)
They are not hollow.
They are from porcelein I think.
My first idea was some kind of high voltage isolator
Can someone explain me what this is, and for what it was used

Much appreciated.

Thank you.

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