monkeyNeilMACHINE 1
Backplane: CN-0F1318-13740-52N-00ZI Rev A01
Mobo: CN-0D8266-13740-52S-005B Rev A02
MACHINE 2
Backplane: CN-0F1318-13740-4BI-005U Rev A01
Mobo: CN-0D8266-13740-4BI-00T7 Rev A02
Both servers have the same parts. The part numbers are right after the country code - CN=Manufactured in China - 0=Nothing D8266/F1318=Part number.
The D8266 is the second revision of the system board, so you don't need to replace anything to put in a PCIe riser. Any of the part numbers you have found for the PCIe riser should work.
Thanks