I asked Glenn Hauser about this, he pointed out that the 810 transmitter is on the other end of the island.
Checking the RHC schedule, I see 13740 is in use at this time, which is 2 x 6870. So perhaps a subharmonic?
I did notice generally strong signals from RHC last night. 6870 varied widely, however, from barely audible to S9+ at times.
Checking the SDR recording, 6870 went off the air at 0459, 13740 is scheduled to close down at 0500, so that seems to be a good fit.