This is what 2A Armament says :
 
 
All the same great styling of our popular titanium gas blocks but in black oxide finished steel. The low profile design works well under hand guards, and anywhere else that space is at a premium.  A basic two set screw design ensures a secure fit, and an easy installation. Blue locking compound is recommended.

    Made from US sourced 4140 steel
    Internal bore:  .625, .750 or .875
    Length:  1.0 inch
    Weight 1.35 ounces
    Blasted and black phosphate coated
    Low profile design
    Includes two stainless set screws (spacing between set screws is .47)
    Engraved logo on front
    Product of Idaho, MADE IN THE USA
    .750 bore, SKU 2A-STGB-1
    .625 bore, SKU 2A-STGB-2
    .875 bore, SKU 2A-STGB-3