OA (1910 to 1928) Previous
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The earliest OA meters were simply an improved version of the previous Type C meters and are of almost identical construction (see first picture). The first picture is of the top-connected style, and also shows the first-generation (embossed) metal cover used on the OA meter (Bottom-connected OAs also exist with this cover). The top-connected style was soon discontinued in favor of the bottom-connected style, and the next major change was to consolidate the meter data onto one nameplate (earlier OA meters have a nameplate on the base as well as capacity markings on the register). Most of the OAs in existence will have this plain metal cover with a small nameplate on front. The final major change was a redesign of the internal frame for better access to the internal connections. Most bottom-connected OA meters are wired with the line terminals on the left side, but some earlier ones exist with the line terminals on the right side (some OAs also have blank terminal covers). A few high-capacity OA meters exist and have a smaller terminal box and cable terminals.
 
OA, top terminals
Westinghouse Handbook on Watthour Meters
OA, glass cover
Mitch Goodwin collection
OA, metal cover
Pete Mallet collection