SBC 350 cylinder head?

Passenger side cylinder head on a 350 SBC.
Anyone know what tool (and who make one) that fits the pipe plug on the passenger side cylinder head where a temp gauge sending
unit might fit?
Its some type of square drive, similar to a 3/8ths ratchet extension , but smaller in size. ----------- Elbert snipped-for-privacy@me.com
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Use a pice of 5/16 key stock and a pipewrench
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