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I think you're on the right track. According to Simon and Schusters, matrícula de un auto, could be a registration number or license plate. In the context that I've seen it used on the web though, it's more often the latter. For example:
Cambio gratuito de matrícula
Cambio gratuito de matrícula:
A los clientes de la marca que lleven su vehículo a cualquier establecimiento de la red de concesionarios Ford se les regalará la nueva placa europea.