OA prepayment (1910 to 1928) Previous
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The first OA prepayment meter superseded both versions of the Type B prepayment meter. The coins were inserted at the top of the meter and accepted by operating the lever at the top of the meter. The mechanism for operating the switch was spring-loaded and wound up as the lever was operated. The round window at the top showed the number of the quarters to the user's credit. Later on, this meter was replaced by a prepayment attachment that could be fitted onto a standard OA watthour meter (right picture). This was a condensed version of the prepayment mechanism used on the earlier OA prepayment meter, except that a credit dial is used instead of the standard kilowatthour register. If desired, a set of dials could be fitted to the mechanism that were visible through the disk window showing kilowatthours used by the customer. The prepayment attachment was operated by simply turning the knob as the quarters were inserted into the coin chute.
 
OA Prepayment meter
Westinghouse Handbook on Watthour Meters
OA Prepayment attachment
Handbook for Electrical Metermen