did all euro pistons come with these holes in the (i presume) oil control ring groove?

They give the oil that the scrapers collect a place to go on the downstroke, since the skirt blocks it right there.

so...all euros have them? feeble memory, us spec has them too?

Well, my memory is possibly worse than yours but I have the KS euro flat tops in my 2.0L....I remember seeing some holes similar to yours but can't recall if they were in the same pattern or if there were as many. So, I think so! I don't seem to have any photos from my rebuild without the rings in the pistons.

I have a new set of Mahles (I think). Holes yes.. in the middle of the groove only.

That pic is a KB piston.