Looks like you are close to correct Quix.
Japan has specific national marine regulations for heavy traffic areas. These differ from international regs.
I posted that this flag was ,I am towing a submerged object. That's what it means in my area.
But in heavy traffic areas it is like an escort vehicle for wide loads on the road.
I actually means. There is a big bugger behind me and he has the right of way, no matter what you think.
I didn't know that and now I do.gary