There are examples of several early adders in the collections of the Conservatoire Nationale des Artes et Métiers in Paris. The Computer History Museum in California and the Science Museum in London have collections of adders and adding machines. Some sources choose to include small adding machines under the name adder.
D. Baxandall, updated by J. Pugh, Calculating Machines and Instruments, London: Science Museum, 1975.
P. A. Kidwell, “Adders Made and used in the United States,” Rittenhouse, 8 #3 (May 1994), 78-94.
Robert Otnes, “Sliding Bar Calculators,” ETCetera #11 (June 1990): pp. 6-8.