This one is the Sterling "Correntator" made by Bergmann (CBR) in the 1930s, serial no 006800. It measures 6.4" x 4.7" and has the flip over panel for subtraction. It is housed in a leatherette wallet (somewhat distressed!) and has a stylus which has a propelling pencil on the other end.
Best regards
David M Riches

Source: David Riches's Collection

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