This is an 5/8" UNF threaded adapter to attach the 1/2" internal diameter brake servo rubber vacuum hose to the inlet manifold.
5/8" UNF is a suitable size if you are plumbing a servo system from scratch as well as being the thread size that many vehicles have as a pre-existing tapping in the inlet manifold (this is covered by a blanking bolt if a servo was not factory fitted)
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