From service manual for HP 34401A DMM.
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>>formatting link
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>> Pins 12 and 13 have a little triangle next to the pin; pins 1, 2, 15, 16 have
>> what looks like an ?n?; 3 and 14 have an ?x?.
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>> What?s the meaning of these marks?
>>
>> Thanks.
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It looks like they are saying these pins go to the AC ground bus.