Purpose of these holes?

[Jac Mac]

You have to remember that without those holes all the airflow has to pass thru the main venturi, when you have a wider throttle plate setting at idle it should be obvious that the amount of air passing thru the venturi increases which in turn increases the signal at the main venturi and if high enough initiates fuel flow from the main venturi, those four holes & the resultant air leak kill off that signal at the main venturi and allow a more precise idle adjustment of the idle circuit in the normal fashion. In DC Dougs case if his cam & engine build etc is of less duration than whatever the carb was originally set up for then they are probably unnecessary & could be blocked off & or replaced with an idle ezi setup if it still requires help in this idle area.
One of the earlier posters summed it up with ' it delays the initiation of the main fuel circuit' or words to that effect'.