So, hands up who knows their bouys? Yes, I thought you did! In that case please tell me what kind is this (and no I don’t just mean a cardinal mark), which of the 4 cardinal markers is it? And for a bonus point… what does this particular type of buoy signify?

Marks the west side of an obstruction to navigation , part of the international buoyage system. dad
Got it in one!
It’s a westerly cardinal marker marking the most westerly point of an underwater feature.
Perfect.