Types CP-3 (1906 to 1908), CP-4 (1908 to the 1920s?) Previous
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The CP-3 was a redesign of the CP-2 using the Form 4 prepayment device instead of the Form 3 device. The CP-3 had the knob mounted on the prepayment mechanism, but any strain on the knob was prone to damage the mechanism, so it was soon superseded by the CP-4 which had the knob in the cover and had a notch on the back that operated a lever on the mechanism. The space above the register in both versions was for a scale showing the number of coins remaining to the user's credit. CP-4 prepayment
Electrical Meterman's Handbook