This is a Samson Manufacturing Corp. Rail known as the Keymod, model KM-4-KIT.
The Samson 4" Picatinny Rail Kit will attach to any standard KeyMod hole. KeyMod Evolution series has slots at the 3" 6 and 9 o'clock positions. Includes 1-4" rail for Samson Evolution Rails.
Features:
Samson KeyMod 4 Rail Kit for Evolution rails;1913 Picatinny Mil-spec;Includes 1-4 rail