This product is either discontinued or we cannot get that from suppliers.
QNAP TS-873-4G 8 Bay Diskless NAS, AMD R-Series RX-421ND quad-core 2.1 GHz processor, Turbo Core up to 3.4 GHz, 4 GB DDR4 (2x 4 GB), 8x SATA 6 Gbps (3.5/2.5-inch SATA hard disk drives, 2.5-inch SATA solid state drives), 2 x M.2 2260/2280 SATA 6Gb/s Slots, 2x PCIe 3.0 x4 Slots, 4x GbE RJ45 Ports.
The TS-873 supports the innovative QM2 PCIe card that allows for adding up to two M.2 SSDs. You can configure SSD caching, or create a RAID 5 tiered storage along with the two M.2 SSDs in the NAS*.