The substrate material is made by the same vendors, and using much the same process as the kinds of laminates people put on the kitchen countertops, etc. Micarta, for example appeas to be a major OEM in both the countertop and PC substrate (and other industrial insulator) markets.
The cloth pattern doesn't look like a "normal" glass-epoxy PC board to me. The "weave" is much too open. And it is so remarkably regular that at first I thought the pattern was printed on.
And the appearance of the shear/break line looks more like paper in the middle than layers of fiberglass mat or cloth. I have laid-up much more of each in my lifetime than I care to remember. Granted it is difficult to tell from a photo.
From some Googling I did yesterday, I learned that a thin layer of fibreglass on either side of a paper core is one of the kinds of substrate material available for cost-sensitive products (such as those amplifiers).