Activity Tiers

Lately I've been looking at Activities and I have noted that the ActivityDefinition is actually a linear list of Activity Tiers. Is there a way to identify/filter activities that belong to the same base/parent Activity (ie getting all versions of Omnigul Nightfall), and if not, would it be possible to get a parentActivityHash variable for this purpose? The only way I can to do filtering at the moment is comparing activityDescriptions, which for some of the older (likely no longer used) activity tiers, doesn't always work because they may have a different description.