Super Gas Block with Black Oxide coating - The bore of the SGB is machined to closely fit around .750” diameter mounting bosses found on most M4 Carbines and AR-15 rifles.
This is a stainless steel gas block from Geissele, and custom blackened by Potomac Armory with Black Oxide. Very comparable to the Nitrided gas block from Geissele: New in with factory instructions, roll pin to drill and pin, and screws to set (use one or the other) and gas block pin for tube.
Note: Black Oxide may have small cosmetic inconsistencies. That does not affect the coating, and the metal is stainless, so you are doubly protected.