Curiosity having finally got the better of me, & please forgive & redirect me if this has been covered previously . . .
Is there a history to the wood shim between the chassis uprights & rear bulkhead?
Was it always there, or introduced at some point to address a structural shortcoming? Why only on the outer top hat flange & not boxed?
I can envisage where there might be some advantage to reducing local stiffness. Also where an increase in stiffness might benefit.