Gunther: "HP Audio Oscillator" ????

PLEASE Be Specific, model number ?
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HP and Office machine mfg. used a proprietary 3-pin Oval connector in 1950s and 1960s.
The HP Audio Oscillators "bridged" this period --- so the connector changed -- deoendung on mfg. date.
The Oval connector is sometimes identifued as AC-3G, and the plug as PH-163
There are TWO cable sets:
Volex (Belden) P/N: 17280 10 B1 ;
Belden P/N: 17952 (Reverse polarity, Line/Neutral swap)
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STANDARDS : Beginning in 1970, the International Electrotechnical Commission (IEC) released IEC 320
https://en.m.wikipedia.org/wiki/Inte...cal_Commission
as its first attempt to Globally standardize Appliance Power Connectors (on appliance)

A separate effort has been working to standardize the commercial AC power voltages
and its power pkugs/sockets.
https://en.m.wikipedia.org/wiki/AC_p...gs_and_sockets

IEC 60320 Appliance couplers for household and similar general purposes (Today's standard).
https://en.m.wikipedia.org/wiki/IEC_60320
Reference Chart
http://www.stayonline.com/reference-iec320.aspx

greg