Offenhauser 6227C-HD Intake Manifold.  It is the C Series Intake Manifold without the heat riser and bolt holes.

Add some muscle! Offenhauser C Series intake manifolds fill the gap between stock and competition setups for 4 and 6-cylinder cars and pickups. They improve torque in the mid and upper ranges, with power bands that span from 2,500 rpm to more than 7,000 rpm. Smaller applications use Holley 5200 or Weber 32/36 Series 2-barrel carbs. Holley 2300s can be used for competition with an adapter. Six-cylinder, 4-barrel versions can use square bore Holley or Carter carbs. Emission-control provisions can be blocked off for competition. Ford 240-300 6 Cylinder Manifold through 1981.  The Dual Port and “C” Series accept the Carter or Holley STD bore 4bbl carb.  Carb size recommended for “C” Series 500 CFM and larger. Use Holley No 8007 (390 CFM), Carter No 9400, or Carter No 9500 on Dual Port – Remove kick-down arm for linkage clearance.  Carbs clear “C” Series manifold without mod.  Offy universal linkage accessory kit is recommended in most installations.  These manifolds on a stock 240-300 C.I. engine can increase HP output by 70 HP!  Dyno tests on a Ford six with Dual Port manifold, headers and cam produced as much as 115 HP over the stocker!Intake Manifold, C-Series, Aluminum, Natural, Square Bore, Ford, 240/300, L6Openings in rear of manifold are 3/8″ & 1/4″ NPTPlease note that this is an extremely popular intake and it sells out very quickly. If this item is listed on backorder, the estimated ship date is currently 8-10 weeks.