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In the meantime I have studied the specimen concerned. When I saw it, it was immediately clear to me that it is not a pallopterid, but a specimen of the family Piophilidae, subfamily Neottiophilinae. It is a female of Neottiophilum praeustum (Meigen, 1826); an ectoparasite in birds' nests.
Although it has been recorded first from The Netherlands only in 1973 (Oosterbroek), it has since been collected at several localities in The Netherlands. Although I have no good overview of the data of this species, I think it is not uncommon in The Netherlands.

Jan Willem