I think that if you assume product engineering teams don't want to verify any more changes from a known good thing as possible you can make some pretty educated guesses about these.
Guess 1(very confident) - Each node is a slight re-spin of the ASRock Rack D1541D4U-2O8R
The pics of the 10G PHY show that it's an Inphi/Cortina CS4227. Of the few commercial options ASRock Rack has for the D1541 only the D1541D4U-2O8R uses that part. The commercial board exposes all 6 SATA ports, 2 M.2, and adds an LSI 3008.
The commercial M.2 ports are adjacent to the PCIe slots and the lower one is set back a bit. On the mystery board the M.2 is near what appear to be OCP slots and on the bottom node there appears to be an unpopulated second M.2 header below and set back from the populated one. My assumption is that whatever engineer was tasked with building the mystery board started by just slapping 2 of their existing designs together, changing the PCIe slot out for an OCP, and removing whatever unneeded parts didn't fit anymore.
That doesn't explain the 8643 plugs, the LSI3008 is a pretty big chip and the mystery board doesn't have any big heatsinks for it.
Guess 2 (very confident) - Each node is totally independent
Building a custom board for a datacenter based around a low-power SoC is kinda weird except for one sorta niche usecase. They let you offer totally discrete dedicated servers with reasonable power for a pretty modest power usage. OVH lists dedicated servers in their locations based on the D1541, so business-wise I can't see any other reason to go through the cost of a custom system except that. Also, in the ebay photos you can kinda make out 2 headers labeled PORT80_A1 and PORT80_B1, which I'm assuming are for connecting LPC motherboard debuggers. If these were a shared system I don't think they would have two low-level management busses?
Guess 3 (very confident) - RAM is DDR4
I'm guessing these were sold at least in part under the OVH ADV-STOR-2 SKU, which lists DDR4 RAM. The commercial board from before is also DDR4 only.
Guess 4 (pretty confident) - The M.2 are PCIe
I'm guessing these were sold at least in part under the OVH ADV-STOR-2 SKU, which has an option for a 500GB NVMe SSD.
Guess 5 (less confident) - The FPGA is the BMC
If I squint enough, it says its an Artix-7. The Artix-7 is the cheap version of a Kintex-7. The OpenBMC has an existing port to the a Kintex-7 eval board, so maybe if you cut out unneeded features you can make a basic controller for 2 nodes fit?
If it is an OpenBMC-based controller it will at the very least have IPMI support and might go all the way up to KVM and a web interface, which would be neat.