Thanks for the input,
to comment briefly ...
Yep, that M.2 connector really is for SATA (among other things), got a proof - close up of system board, form that eBay seller. It's not the only component that moved elsewhere, from the concept board/prototype to production board. The M.2 SATA options is still there (above the front facing edge connectors), though only for full-length (22 mm x 110 mm Form Factor) M.2 SSDs.
I may have jumped some conclusions, about what type of modules can be attached to those two front facing PCIe x8 edge connectors. So the reference to "storage", as one of the possible module types, was actually valid in the OCP document. Given how those connectors are marked on the board (NETWORK MODULE 0, NETWORK MODULE 1), who could blame me.
Nevertheless, I'm absolutely sure that a regular PCI Express SAS/SATA HBA or RAID controller card cannot be attached to any of those two edge connectors. Once upon a time, a physical PCI Express connector were designated for standard PCIe cards (video, storage controllers, networrk cards, you name it ...), but nowadays they are used for all kinds of add-on modules and daughter cards (for being cheap and abundant), with proprietary wiring, of course.
Funnily enough, your board is equipped with that elusive "EXPANSION PCIE X4" (as it's a different Silicom uCPE model), while the board in ATT-V150 isn't (same goes for dual onboard 7-pin SATA connectors). That's the one PCIe x4 that I thought I would use, to connect the SAS/SATA HBA card, - with a little help of flex ribbon PCIe cable and some standoffs for the controller card (sans ATX bracket).
Come to think of - I examined closely the ATT-V150 board and the pre-production board of Silicom uCPE Extra Small, nothing seems to be missing, except the physical edge connector (all semiconductors and discrete elements are in place, albeit a bit shuffled around the board). Might as well purchase a straddle mount PCIe x4 and solder it onto board - the worst that can happen, is that it won't work.
