Stefan Wolfe wrote:
One of the enduring sub-codes is "73" meaning "Best regards." Hams use it on
voice, as well. It has become traditional jargon.

Quoting the "The ARRL Operating Manual" with them
in turn quoting an 1857 publication: "At that time,
73 meant 'My love to you'."

When I was in high school, I received a QSL card
from a YL ham. She had scratched out 73 and written
in 88 (love and kisses).
--
73, Cecil http://www.w5dxp.com