A variable-volume, constant pressure plethysmograph.  Undated photo from the South Australia Medical Heritage Society website  and probably constructed around 1970. Manufacturer or builder was unnamed.  The following description was included:

“The picture above is a more recent example of a variable volume plethysmograph. This type of box was designed in 1960 by Dr Jere Mead, and is similar to the box used by Haldane and Priestley.
The subject breathed air from outside of the box. The resulting change in box volume was recorded by a spirometer or calculated from a pneumotachograph. The pressure at the mouth could be measured directly, and the amount of air breathed was recorded by a pneumotachograph.”