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The numbering convention is consistent with the pinout of old tube sockets. Viewed from the bottom where engineers and technicians actually do the wiring, read pin numbers starting with #1 just clockwise from the marker in the base. Tube manuals and schematics gave the bottom view.
The numbering convention continued with transistor (TO-5 package) and the bottom view in transistor manuals.
ICs have the same numbering system but are shown with a top view on data sheets so pin 1 is counterclockwise from the package marker. I suppose this is because techs probe from the top and IC test clips attach to the top of DIP packages. It can be confusing because a transistor in a TO-3 package has a bottom view in its data sheet, but an IC voltage regulator in a TO-3 package has a top view.
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