I contacted a Dellorto carburettor specialist who seemed very knowledgable.
He said that these were air intake holes & they help to control the mixture providing the fine tuning in between jet sizes. They are not interchangeable being part of the carburettor body and therefore not available as a spare part.
He went on to say that the size would also be unique to your engine/model of scooter so you could buy say a 20mm carb and re jet to suit your machine but these hole sizes may be incorrect
Thanks