Hit the nail on the head there
the standard aussie sixpack carbs has 1 big 1 small progresion hole like an IDA, but a load of street features due to the desire to do circuit racing in saloon cars
Seems to be quite a few different types & revisions, as Bewy pointed out.
Not sure why Weber or Dellorto couldn’t have just used a transition slot for ‘street’ carb. I’m sure someone has tried modifying a Weber with a T/slot.