G'Day, mate!

Yes, indeed, they DO look like stiff cardboard, because that's what they are!
I'm told by my people that they were made of compressed wood fibre, treated in
the compression process to resist disintegration due to the elements, etc.

The Canadian Army used what was probably the same type until we discovered
aluminum.

We might be able to provide more technical information. Contact
helen.holt(at)cmcc.muse.digital.ca

Regards to Liam, Helen, Paul, etc.

Harry Needham
Director, Programmes and Operations
Canadian War Museum