Found thishttp://www.ac6v.com/73.htm "QRPers sometimes sign off with 72 indicating they may be a mite short on power for a full 73! But Kevin Cozens writes "In one of the magazines I was getting for a while from one of the QRP clubs (can't remember if it was from the Michigan QRP club or the G-QRP club) I learned of the use of 72 for the first time. Their use of 72 was based on the idea that "QRP operators do more with less". If you add that to the page you will have both a QRP as well as the QRO operators view of 72."
Also this on 73http://www.signalharbor.com/73.html
"Many amateurs already know that "73" is from what is known as the "Phillips Code", a series of numeric messages conceived for the purpose of cutting down transmission time on the old land telegraph systems when sending text that is basically the same.
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