Grant,
I have seen two types of construction. One type is basically a 10"x10"x1 3/4" piece of plywood. This is the type that suffers de-lamination of the plys. (Brian may have detail pics of these)
The type in Brian's picture is essentially a 10"x10"x1 3/4" hardwood block that has a 5/8" wide x 15/16" groove cut into the end-grain. This groove then has a 5/8" x 15/16" x 10" piece of wood glued into it to discourage warping of the main block.
Attached you should find a couple of picture. Unfortunately it is dark in the shed and the pictures were taken with a flash.
Colin
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