I posted this also over on QRZ and got some interesting replies; this all led me to interesting stuff including some history of the experimental "X" callsigns (which practice seems to date from around 1914) and to solving a curiosity about W9XC. I'm going to write up what I've found and post it on my website.
Here's the curiosity: A couple of years ago I also had searched for "W9XC". One site (and one only) that popped up had a W9XC in Iowa at 1040 Kc on a list of AM stations:
"U. S. AM Stations as of 1942" http://jeff560.tripod.com/1942am.html
I thought it must have been a typo, "WPXC" or something. Today, I found the Radio Annual lists of all BC stations over many years:http://www.americanradiohistory.com/Radio_Annual_Master_Page.htmhttp://www.americanradiohistory.com/Archive-Radio-Annual/1939/Radio-FM-RA-1939.pdf
Lo-and-behold, W9XC is there, in Mitchelville IA, in the sections on "Experimental" or "Developmental" stations, in each list from 1939-1942. So, no typo, but a bona-fide experimental station. BTW There are other very "ham-looking" calls in those RA lists too.
According to K9STH over on QRZ it appears that in the late 1970s the FCC opened up two letter calls for request, and at that point included the X calls for ordinary ham callsigns as well. This jives with the Callbooks I have, but I still don't know the year that this happened.
Anyhow, nice to know some more history of my own call, and others may find the X-calls history interesting too.
- Les, W9XC