'XSR' is the 'screen name' of WC2XSR/13, issued to W5JGV, a member of the Six Hundred Metre Research Group. It is to my knowledge the sole one of the group licensees presently radiating anything; it is on 166.5kHz, power of 400W from Louisiana. Ralph has an interesting website about SHMRG, his station and things LF.
In between the readable 'XSR' characters, one can see a little 'blip' - this is normal-speed CW identification.
© Steve Dove, W3EEE, 2003