HP 689911-071 branded ECC RAM modules which use Samsung M393B1K70DH0 - CK0Q8 DIMMs.
1.5 V / 1.35 V
8GB per module / DIMM

Multiple configurations available (just choose the appropriate number of DIMMs as the quantity to order):

Manufacturer Datasheet:
https://semiconductor.samsung.com/dram/module/rdimm/m393b1k70dh0-ck0/

DDR3 8GB 1600MHz ECC RAM 2Rx4 PC3-12800R-11-11-E2-D3 Memory - Multiple DIMMs.