Well, you're all right about it being some kind of adapter, of course.
You hit it exactly! The plug is a 3-pin 5A type, the outlets on each side face will accept a 2-pin non-grounding 5A plug, and the front face will accept either a grounding or non-grounding 5A plug. The top hole (earth/ground) and the bottom two are used for the grounding plug, the other two for the non-grounding plug.
It is an unusual style of adapter. The majority of adapters of this type had just the three holes on the front for a 3-pin grounding type plug.
This picture highlights the fact that the pin spacing on grounding and non-grounding plugs is different, so a 2-pin plug won't fit the phase & neutral holes of a 3-pin outlet, unlike the American equivalents.
That white adapter of yours looks like the plug and front socket are the 15A types.