Yes, SWL QSLs were HUGE in the 1950s and 1960s.

Popular Electronics and Electronics Illustrated even assigned "call letters"
to SWLs which they
used on very attractive cards.

I still use mine occasionally and even though I am a ham I still enjoy
sending out SWL cards. Stations
seem to like getting them too.

Do a search on the Internet for "WPE" and you'll probably see a lot of the
cards we used to
send around.