Hi graham, it's 6mm hardwood marine ply which gives great strength and stability. Try to buy marine ply as the glue for obvious reasons is not affected by moisture.
Don't be tempted to use individual hardwood planks as you will have a hell of a job keeping it all in place.
I'm old enough to have worked on a farm that used the ransoms thresher machine although I was very young at the time

Mechanical engineer.
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