CQ Amateur Radio has relaunched their historical Monitoring Station registration scheme for radio listeners. There was also an online database with CQ/PC registrations.

More info:
How to Apply for a CQ Monitoring Station ID by Richard Fisher, KI6SN, 2014-07-18
cqpluscqms.blogspot.com/2014/07/how-to-apply-for-cq-monitoring-station.html

This is my certificate, with "call letters" SVCQ1XV.