There are two big boards on that page, there is a four PCIe slot board and a five PCIe slot board. The spacing on the '4' is a lot wider than on the '5' (it is also non-standard so a normal enclosure will not work).

"PLX88096 PCIe 4.0 Switch GPU Expansionbackplane 4/5 Slot SFF-8654 SlimSAS"

Is the description on the page. That C-Payne switch is a beauty, they make some very nice hardware.

This is also interesting:

https://www.utran.com.tw/product_1701777.html