That is the foundry logo. GM for years has sub contracted many different foundries to do cast aluminum items; mainly (but not restricted to) intake manifolds.
The biggest one was Winters who cast many of the aluminum intake manifolds and cylinder heads for Chevrolet performance engines in the 60's and 70's
But it does kind of look like the Wisconsin Aluminum Foundry logo
I stumbled across these for a "Wisconsin Foundry and Machine Company" in Madison. Don't know if there's any connection, other than the one common design element.