The DQ2 is a high voltage half wave mercury vapour rectifier. The 866A is given as an equivalent but that rectifier has a maximum cathode current of 0.5 Amps and the only details found for the DQ2 give a lower figure for cathode current.
The rectifier will produce damaging UV radiation and must be screened.
Brown Boveri and Made in Switzerland on the base cap.
The UX4 base and a view into the cathode cylinder.
The heater/cathode is a wire mesh.
The classic envelope is 50 mm in diameter and, excluding the UX4 base pins, is 136 mm tall.
Reference: Data-sheet. Type DQ2 was first introduced in 1938. See also1938 adverts.