Eventually retrieved item from post office: it's a USB 3.1-to-SATA adaptor cable that lets me operate an elderly external SSD at twice the read/write speed as USB3.0-to-SATA interface. I already had the capability of such a connection speed, but it needed a Thunderbolt 3-to-Thunderbolt 2 adaptor plus a Thunderbolt 1-to-SATA interface & a Thunderbolt 1 cable. This is much less fiddly.