I know some Old Timers who insist if you're going to use it for voice, you should say each number separately, as in "Seven Three, Old Man".
I, personally, don't have a problem with hearing someone say "Seventy-Three", but the plural form just grates me.
Since my YF (wife, to non-radio people) edits the club newsletter, and is always looking for articles, one of our members sent in a scan of a page from the April, 1935 edition of "QST", the magazine of the ARRL.
Since the scan wasn't very high quality, I cut-and-pasted the print into a Word document, and then cleaned it up by hand.
On the Origin of “73”
The December Bulletin from the Navy Department Office of the Chief of Naval Operations contains some extremely interesting information about "73."
The origin of "73" as the telegrapher's conventional signal of greeting has been ascribed to a dinner given to Andrew Carnegie on his 73rd birthday by the Order of Military Telegraphers. The dinner referred to was given on November 27, 1908, and the signal "73" was played upon in connection with his 73rd birthday. However, investigation indicates that the term "73" was used for many years prior to that time.
The following material, believed to be authentic, is quoted from the "Telegraph and Telephone Age," dated 1 June,1934:
"It appears from a research of telegraph histories that in
1859 the telegraph people held a convention, and one of its
features was a discussion as to the saving of 'Iine time.' A
committee was appointed to devise a code to reduce standard
expressions to symbols or figures, This committee
worked out a figure code, from figure 1 to 92. Most of these
figure symbols became obsolescent, but a few remain to this
date, such as 4, which means 'Where shall I go ahead?' Figure
9 means 'wire,' the wire chief being on the wire and that
everyone should close their keys. Symbol 13 means 'I don't
understand'; 22 is 'love and a kiss'; 30 means 'good night'
or 'the end.' The symbol most often used now is 73, which
means 'my compliments,' and 92 is for the word 'deliver.'
The other figures in between the foregoing have fallen into
almost complete disuse."
Mr, J. L. Bishop, Chief Telegrapher of the Navy Department Communication Office, quotes from memory the signals that were in effect in 1905:
1 Wait a minute.
4 Where shall I start in message?
5 Have you anything for me?
9 Attention, or clear the wire (used by wire chiefs and
13 I do not understand.
22 Love and kisses.
25 Busy on another circuit.
30 Finished, the end (YA) (Used mostly by press telegraphers
to denote the end of a story or closing out).
73 My compliments, or Best regards.