found a mushroom shaped cement type marker can anyone identify what this is?

[Len]

Here is a long shot.--Perhaps some kind of a corner boundary property line marker?

 

[PlaneDiggerCam]

CanadianBottles is 100% correct. These are stoppers to big chemical demijohns. It's some sort of composite pottery-like material that is designed to be unaffected by whatever was in the demijohn.

I second this. These are demijohn / large stoneware jug stoppers.