How and when did "xo" come to mean "hugs and kisses"?

How and when did “xo” come to mean “hugs and kisses”?

About 40 years ago, I indulged it such nonsense, but the hugs part was not in use (this doesn’t mean that it had never been in use before). Things like S.W.A.K. were big. You have to remember that we actually wrote letters, put them in envelops and mailed them.

K903, here’s what I found searching online (though I was somewhat astonished to find that this question hasn’t been answered previously on this board):

This is from the following website:

Link to Oxford English Dictionary entry:]

This is a less than satisfactory answer. Maybe a Doper with a background in semiotics/linguistics could help?