"Contex" Half-keyboard Adder, Model A, S/N 73053
Despite its appearance, the Contex Adder does not operate in base five. This machine uses half of a keyboard in order to simplify its internal mechanism. To add with numbers greater than five, one must use addition(to get 7, add 5 and 2, ect.).
"Contex" Model 10, S/N 434 005 097;
""Contex" Half-keyboard Adder, Model A, S/N 131698; Source: John Wolff's Web Museum
"Contex" Half-keyboard Adder, Model B, S/N 911428; Source: John Wolff's Web Museum
Front of the Model A Adder
Logo on the back of the Model A Adder
Writing on the bottom of the Model A Adder
Front of Model 10 Adder
Back of Model 10 Adder
Writing on the bottom of the Model 10 Adder
Model 10 Adder with a clear case, thanks D.Barcelona
Also check out the Monroe 'Mini' Model L.