Contex calculators were first introduced by Carlsen Brothers in Denmark in 1946 and the displayed half keyboard version would have been one of the first sold. The company exported to more than 100 companies, including Australia, with Sterling versions to 1966, and then decimal.

One of the first Contex calculators, the Half-Keyboard Adder (Model A), used Leibniz step cylinders to perform simple addition and would cost $50 in the early 1950’s. This model was manufactured in 2 different types: a Sterling version and a decimal version as it was commonly used in monetary trade and small stores. The idea of a hollow half keyboard as opposed to the enclosed shape of other devices, such as the arithmometer, was that it made the device lighter and this idea was extended when the metal outer casing was replaced by plastic. The half keyboard had keys only up to 5 so that numbers above 5 had to be handled with two entries eg 9 is equal to 5 then 4. This halved the size and weight and cost. Later production added electric motors to increase the speed of operation. Subtraction keys and zeroing buttons for each section were also added to increase versatility and reduce errors. In the late 1960’s Contex calculators were capable of automatic multiplication, addition and subtraction and semi-automatic division with some including printers for answers.

The predominant use of this Contex on display would have been adding sterling currency. Starting from the right, the levers were used for ½ and ¼ pence, the next row would be pence, followed by shillings up to nineteen, and pounds. On the far left there is a zeroing lever for the entire mechanism.

Source: The University of Queensland | Physics Museum (March 2025)  (See https://physicsmuseum.uq.edu.au/contex-adding-machine)