Richter Spielgeräte

This may look like something from the Lost island, but it's one of a pair of large concrete acoustic echo units facing each other on the edge of the children's playground at the north end of Ravenscourt Park.

In large (and maybe zoomed too) you can see the company name and a graphic showing how to use them in the centre.

Judging by their website, this is one of many interesting and well-designed play units designed by German company Richter Spielgeräte.