Complete HK416A3 2 Position Gas block complete with internal parts.
 
Specs:
HK416 Slimline Gas Block
no bayonet lug
no sling loop
Fits BRN-4, MR556, MR223
for 14" & 16" barrel lengths
 
Spring, roll pin, selector and internal pin might come separate from the gas block and have to be installed by the buyer. 
 
Made in Germany by HK Germany.