Strange wooden block, possibly connected to RAF flight logs?

Hi guys,

What is this?? I acquired it from an ex RAF pilot, he couldn't remember what it was for but he thought it was something to do with Decca flight logs? It came with a load of flight logs and a box which is marked 'Chivenor Pattern Decca' with some codes on it.

Anyone any ideas? it doesn't look homemade, the 3 black hooks are attached to a spring inside, they can be pulled up but spring back again. The circular wooden piece with spikes sits in the large hole I think.

Thanks in advance!

Sonya