The usage of "73" for such a purpose comes from the Phillips Code, originally devised in the era of telegraphs to speed up transmission of common messages by mapping them to numbers.

"73" mapped to "best regards" or "my compliments" and was intended as a general valediction for transmitted messages.

It is still used for that purpose today in morse code transmissions and often makes its way into everyday correspondence among hams as a form of jargon.


So 73 means message transmitted - is that correct?