Call History

I accidentally came across a card on a French Auction site that Ann M. Carson, W3XYL sent back in 1948.

I then tried to find out more about Ann M. Carson and in the process ran into  some very interesting stuff.
Calls with an "X" for the first letter of the suffix didn't exist until sometime in the 70's, except for a few Experimental stations.
It has the SWL inserted between the 3 and the XYL.

I figure that Ann was a SWL and probably had cards made to send to stations she heard as a SWL. The mystery is why in the To Radio slot she wrote in " swl" instead of a call sign.

There is no way she could have heard another SWL, since swl's don't transmit. ? ? ?
So the project now is trying to figure out the mystery of who she was and the real history of the W3XYL card.

This card was sent from Philly to Bois D' Haine,  Belgium   6/11/1948