The Z865W is a small cold cathode thyratron or trigger tube. As this is a gas filled valve the leads are covered with ceramic tubes where a discharge is not wanted.
Side view.
The gettering prevents a clear view of the inner structure.
The thin glass tube envelope is 21 mm in diameter and, excluding the base pins, is 47 mm tall.
Reference: Data-sheet. Type Z865W was first introduced in 1968. See also1968 adverts.
