CQ Amateur Radio (which now incorporates Popular Communications in its digital version) has relaunched their historical Monitoring Station registration scheme for radio listeners. There is also an online database with CQ/PC registrations.
Today I received my certificate, with "call letters" SVCQ1XV.
Read more on CQ Newsroom blog in a posting by Richard Fisher, KCQ6CQ.
Strangely, CQ/PopCom don't mention the Monitor Station Registry offered by Tom Kneitel's company CRB Research in the 1980s. I had applied for that as well in 1985 and they allocated me call letters SV01MRXV.
I have found an interesting account about the WPE Monitor Registration Program , the WDX registration service and the rest of those U.S. SWL registration schemes on the HF Underground wiki.