This is an 5/8" UNF threaded adapter to attach the 1/2" internal diameter brake servo rubber vacuum hose to the inlet manifold and also incorporates a one way valve.
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)
A one way valve is a recommended fitment for brake servos

Typically used to fit remote servo systems such as in the picture (note that you are just buying the manifold adapter not also the servo). The servo kits themselves are commonly available including here on ebay - but they don't include this adapter and valve neccessary to plumb into the inlet manifold.

Suits Classic Mini, Triumph, MG etc