Magpul M-LOK® Aluminum Rail, 3 Slots
From the Manufacturer:
Designed for use with all M-LOK compatible Hand Guards and Forends, M-LOK® Aluminum Rail Sections allow the attachment of various 1913 Picatinny spec rail-mounted accessories such as lights, vertical grips, etc. Machined from milspec-anodized aluminum, M-LOK® Rail Sections also feature beveled ends to reduce snagging and eliminate sharp corners and edges. All hardware necessary for attachment directly to M-LOK® Slots on either aluminum or polymer hand guards and forends is included. 3 Slots, max overall length of 1.6".