New comment by detaro in "Fixed Layout Prototyping – Hexagonal Electronics Breadboard"

Yes, I am sure. "Copper clad" - board with copper cladding on it. Here's a higher-quality picture taken at an angle from a different article where you can clearly see that it's not massive metal, but fibrous board:

https://i0.wp.com/www.pbase.com/daverichards/image/136603883... (from https://aa7ee.wordpress.com/2011/07/24/the-wbr-a-simple-high...)

EDIT: in the PDF, page 8 also has an image where you can sort-of see it.

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