In the GM manuals, it's called a "coolant outlet adapter". If you
want to get a new one, you will probably only find this part at
the dealership. In that case, who cares what it's called, just go
in and have them bring up an exploded view of the cylinder
head and point at the part that you want.
If you are looking for a used part at a wrecking yard....your
description above is enough info for a wrecking yard crew.
They are quite savvy when it comes to engine parts.
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