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This seven pin base originated in America in two forms that are slightly different as the image below makes clear. The smaller pitch circle is the earlier design and is known as the Small 7-pin and A7-8 drawing also as USS7. The base dimension from pin tip to top of base is 1.436 inches (36.47 mm). The later version with a larger pitch circle but the same pin diameters is known as the Medium 7-pin, drawing A7-13 and UX7 and here the pin tip to top of base skirt is 1.68 inches (42.67 mm). The UX7 is also known as the USM7.
Image courtesy Jeremy Harmer. Large PDF here.
As both look identical at first sight, both are presented together as UX7 with clarification within the exhibit text where possible.
UX7 ceramic base.
The pin-outs below refer to the data researched by Keith R Thrower OBE.
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