"COLIN LAMB" ) writes:
Bill Orr had a column in either Ham Radio or CQ with a W10 card. They are
rare, but were issued for some special purpose. I think it may have been
pre war. My memory is fuzzy, but it may have been for shipboard use.

X calls were often issued for experimental stations.

73, Colin K7FM

I wondered if it was a WWII call for some reason. Obviously there was
no ham radio, but wasn't there some CD or emergency service that was
active during WWII, on the VHF bands? I can't say I've ever seen mention
of a W10 callsign, but perhaps it was during that period?

Michael VE2BVW