What socket fits the Catalytic-converter-to-Exhaust-manifold stud?

I’m trying to replace the exhaust manifold on my 2010 Chevrolet Equinox (2.4L 4-cylinder), and there are studs which connect the catalytic converter to the exhaust manifold. I cannot, for the life of me, find the name of the head on these things. I also cannot use the double-nut method since there is little to no working space available to me.

If anyone could identify this, I would greatly appreciate it. Other Ecotec engines of this period had inverted Torx sockets, but this one seems to be some rounded-rectangular head.

I’ve currently got penetrating lubricant sitting on the stud and plan on hitting it with the cold fluid from an upside-down compressed air canister to shrink the metal ever so slightly to help with extraction. Any other tips appreciated 🙂