Mystery on 1370

On September 19, I heard a very weak station on 1370 kHz at 0500 UTC. It was so weak that all I could hear were voices mumbling way down in the mud. I could not make out a single word and would have tuned to another frequency, but right at the top of the hour, there was a high-pitched tone (time signal) followed by weak, but decipherable Morse Code.

Just four letters: TRLB or TRLD. The signal faded badly on the last letter and I am not sure if it was a B or a D. I searched high and low trying to find out what those letters signify, but I have found nothing. Grasping for straws, I thought maybe that the TR was really a C, but I could not find anything on CLB and CLD either.

Since then, I have been returning to 1370 at the top of the hour, but I have not heard a repeat of the Morse code.

If anyone has a clue, I would sure like to hear it!