[Lowfer] Call sign " OF " @ 508.920 KHz

[Andy - KU4XR]

I heard a signal identifying as " OF " Sat night around 1am Eastern time, I think that is 05:00 UTC. The signal had a lot of QSB here, but my noise is awful. My curiousity is; NDB - " OF " in Nebraska, is supposed to be on 510 KHz. Is it possible someone else is using that callsign too ?? That would be unusual I think, but I definitely heard the signal at 508.920 KHz.
Did anyone else happen to hear this ?? BTW; I tuned 510 KHz and heard nothing, but SpecLab showed a carrier trace in the noise.

73 all: Andy - KU4XR, Friendsville, TN.