field_609103a4249a4 is the unique field key for the field. Because the field name can be changed this is what ACF uses to identify the field. This value never changes. You can see these field keys when editing a field group. Top of the page => Page Options, check the box for field keys.
When using acf/save_post if you use a priority of > 10 for your action then you can get the values from the DB by using ACF functions rather than trying to look in $_POST.
