Manfrotto 015 brass adapter with 1/4" and 3/8" male threads on the ends. The stud head itself is a standard (16mm) 5/8" size.

Manfrotto 118 Spigot Adapter, with 1/4' and 3/8' male threads on the ends. The stud head itself is a standard (16mm) 5/8' size. There is a hole in the middle that can be used as an umbrella holder, for example.
Material: brass
Weight: approx. 40 g