The variables for that Joomla event can be found below.
manual.joomla.org/docs/building-extensions/plugins/plugin-events/content/#oncontentbeforedisplay
So the following would be how they're mapped in CB Auto Actions.
var1 = context
var2 = item
var3 = params
var4 = page
For articles the context would be "com_content.article". This would mean your first condition needs to be the following.
Field: Custom > Value
Custom Value: [var1]
Operator: Equal To
Value: com_content.article
The problem with the access check is I believe Joomla's ACL will prevent access and result in a 404 before that event ever fires. So your article would have to be set to Public then you add your access check basically by using the auto action to redirect away and the easiest way to do that is just set the auto actions Access to "All Non-Registered Users" as you've already done. I suppose you'd need a way to flag specific articles for this redirect behavior though. Are they in a specific category? maybe have a specific Joomla tag?