Hi Alwyn, the five-digit numbers starting with 5 which you saw are the IDs of the 'parent station' in the GTFS. A parent station is a grouping of multiple bus stops. So here there is a parent station "Auckland International Airport" (ID 51473) which groups multiple bus stops, including "Stop A International Airport" (stop code 2008)

The "Auckland International Airport" parent station is mapped, represented by the OSM relation https://www.openstreetmap.org/relation/15353894
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            Published using OSMCha: https://osmcha.org/changesets/140435125
            