High level ab initio calculations carried out for the ²A' and ²A'' states of ArNO(X ²) predict a crossing near the T-shape configuration, with the ²A' minimum being slightly deeper. Spin-orbit coupling is included through a model treatment and results in two potential energy surfaces with similar topologies, nearly parallel to each other and close to the averaged non-relativistic surface. These results are used to construct a DIM-like model for Ar_nNO clusters. The lowest energy cluster structures are found to resemble those for Ar_n+1 with NO lying in the surface. The set of major magic numbers (structures of pronounced stability) is also the same as for the Ar_n+1 clusters, and is emphasized further by the detachment of NO, which requires a larger energy than for detachment of a single Ar atom. The relations of the difference between the two dissociation energies and of the Ar_nNO(1/2 3/2) excitation energy to the magic numbers are discussed.