Weston Master III Universal Exposure Meter Model 737 / S141.3: 1956, USA and UK manufacture. Brushed steel cased USA meter (left), the rarer red dial UK model (centre) and standard glossy black enamelled model (right). Note that the American model (left) now uses A.S.A. film speed rating whilst the UK models retain Weston Speeds. With all three models now incorporating the Exposure Value (EV) scale the calculator dials on the two British models have been redesigned. The addition of the EV scale and the new dial design with its red variant are the only differences (on the UK model) between the Master III and the earlier Master II. The rest of the meter remains the same. An interesting detail on the US model (left) is the further addition of a scale for the Polaroid exposure values.
One option for fitted cases for this series of meters is shown here. The back section of this ever-ready case has a recessed compartment to house the invercone and filter for high light levels. The Master IV and subsequent Masters had a redesigned perspex invercone which was protected in a separate 'satchel' type leather case.