Hi ToniE,
I am not completely sure about your implementation.
For “transport association”, the concession (those 49 seem right) seems to be the most logical unit. However, the name “transport association” is a bit confusing, as there are transport associations giving out multiple concessions (e.g. the Vervoersregio Amsterdam (VRA) giving out the concessions Zaanstreek-Waterland, Stadsvervoer Amsterdam and Amstelland Meerlanden) and is a governemental body controlled by the municipalities, while most concessions are handed out by the provinces.
Also, there are currently 9 public transport companies active in the Netherlands: NS (Nederlandse Spoorwegen) for the majority of the rail transport; GVB, HTM and RET for bus, tram, metro in Rotterdam, The Hague and Rotterdam respectively, and Transdev, Qbuzz, Arriva, Keolis and EBS for bus and local rail outside of the three big city centres. They sometimes operate under their own name, sometimes under an older business name (Connexxion for Transdev, Syntus for Keolis), a brand name of their own (like M.NET for EBS in ZaWa) or a brand name of the operator (BraVo for the bus transport in Brabant, split into two concessions run by Arriva and Transdev). A few lines are run from two concessions and by two operators (e.g. line 295 Rotterdam-Utrecht ran under two concessions and by two operators) and line 300 Amsterdam-Haarlem which is one line but technically has part of it belonging to concession AML and the other part to concession H/IJ, although it is fully run as part of the AML concession. Then there’s the “formula’s” like R·NET, which is a “fast bus” brand owned by a concortium of PTA’s and is rand by all metro lines, some fast tram and bus lines of 13 different concessions, but also “buutbus” which is an uncopyrighted formula for local bus services ran by volunteers.
“Search area”: a province is usually a good base, but a concession might span only a part of a province, and there are now two (bus) concessions that span multiple provinces (GD for Groningen/Drente and IJssel-Vecht for the provinces Flevoland, Gelderland and Overijssel) Most of the rail concessions will span multiple provinces and Hoofdrailnet will span all of the Netherlands.
Of course, the province-based concessions will have lines that extend outside of their area, but I think all lines will have at least part in their “home” province - but there might be short trips that run entirely outside the “home” province.
Also, the rail concession Regionaal Spoorvervoer Fryslan en Groningen is partially run by a German transport authority. See (this)[Concessietabel - OV in Nederland Wiki] page for an overview of concessions.