I think this schedule of S06 (0400 each week day morning on 15721 kHz) was first noticed by US west coast listeners a few years back, maybe it was solarix in the #wunclub chat? I have often heard it better on the US west coast than on any European remote, aside from the QRM. The reason I mention that last is because bearing cuts from my location would seem to point away from Europe as the source, with Asiatic Russia or Kamchatka being candidates.
This is the same bearing as the Sunday morning V07 transmission for me, and this V07 is seldom heard (or at least reported) in Europe. This brings me to wonder if maybe both are transmitted from the same facility general area.
T!