According to a poster on the site.
"Alex Ray:
David Briggs yes its Teak.
It was used as a backing on the armour plates for twofold reasons. Firstly to soak up any spalling from the armour plate in the event of a hit, and secondly to give the armour plate the slightest amount of spring,,to help soak up some of the energy of a hit, meaning that the armour plate was better able to take a shell hit and remain unpenetrated."