Glen Zook, K9STH, posted this to the Heathkit mailing list:

In the April 1935 issue of QST on page 60 there is a short article on the origin of 73. This article was a summation of another article that appeared in the "December Bulletin from the Navy Department Office of the Chief of Naval Operations". That would be December of 1934.

The quotation from the Navy is as follows: "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 'line 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 forgoing have fallen into almost complete disuse."

One of the chief telegraphers of the Navy Department of Communications, a J. L. Bishop, quoted 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 13 I do not understand 22 Love and kisses 25 Busy on another circuit 30 Finished, the end-used mainly by press telegraphers 73 My compliments, or Best Regards 92 Deliver

Making any of these numbers plural (73s, 88s, etc.) is incorrect since they are already plural. 73s would mean best regardses and 88s would mean love and kisseses. Those make no sense.

Anyway, the subject of where 73 came from comes up periodically and this article reinforces the "Phillips Code" origin.