Two possibilities, or both, for that inverted feature. Looking at one on hand, the flashing from the molding process in perpendicular to that “hole”. Possible that this feature was designed into the vessel to be where it was pushed out from the mold in production.
Other possibility is that these Ate bottles are hardly unique to the E9–some version was used in essentially every European car produced for 40-50 years. Ate would have specifically configured fluid vessels for each contracted application, but would be drawing off a basic catalog of bottles, then reconfigure somewhat for each application. Many applications (eg Porsche),
use a very similar bottle, but instead of the plastic “bracket” piece plastic riveted to the bottle like in this E9 bottle, used a metal bracket to fasten to desired location. That “divot” served as a locating element (bracket would have a boss feature to fit inside) to keep the bottle stationary/prevent rotation.
