This is the Supermicro AOM-SAS3-8I8E-LP, a dual port SFF-8644 to dual port SFF-8643 converter / adapter PCIe bracket in low profile. If you're building a JBOD or drive enclosure, and you need to connect an external cable to your enclosure chassis and convert it into internal ports, this is the bracket you need. Mount it on your chassis with a low profile PCI bracket slot, connect the external cables, and then connect the internal cables. This also ensures your external cable shielding is properly grounded to the chassis and to your PSU. Fits in low profile cases only.
Please feel free to ask any questions before you order. If you need help choosing the parts you need to build out your drive enclosure, I have many YouTube videos that can help you. Just look up my channel "The ART of SERVER" on YouTube! :-)