This is a Diamondhead USA, Inc. Accessory known as the VRS Rail Section, model 2821.
Diamondhead's VRS 2" Short Rail Section" for Diamondhead VRS (Versa-Rail System) free floating and drop in AR-15 handguards" is threaded for Mil-Std 1913 Picatinny rail applications.
Features:
Attaches to the VRS Drop-in or Free Floating Handguard with included hardware
Mil-Std 1913 Picatinny rail dimensions
Fast configuration changes