This Bruel & Kjaer 4220 Piston phone Manufactured in Denmark is in excellent condition often used in Calibration Laboratory's and can be used in the field for calibrating Precision Sound Level Meters.
Batteries are not shipped with the unit and it will take 6 x 'AA' Alkaline cells.

It produces a tone of 250Hz at 124db +/- 0.2db SPL.
To maintain this accuracy it comes with a special Bruel & Kjaer Pressure Correction Barometer UZ0001.
Calibration last checked to be correct in a Calibration Lab, 2009.
I checked the unit recently and it produced a tone so it is working.