What is this SBC intake manifold?

[Keith Tedford]

Offenhauser also made a similar intake with three different lids. One was like the Chevy two four setup, one with the single four like this one and then one where you could add three webers or similar carbs. Unless the numbers and letters have been removed, the Edelbrock SY1 and the Offy were clearly identified, as was the Chevy intake.
Check this out. It too has the square indent and the long narrow one on the opposite side of the carb flange. http://victorylibrary.com/graphics/SY1.JPG