This part of the IEC 63267 series defines the dimensional limits of an optical interface for reference connectors with rectangular ferrules necessary to meet specific requirements for fibre-to-fibre interconnection of non-angled polished multimode reference connectors with rectangular ferrules intended to be used for attenuation measurements as defined in IEC 63267-2-2. Ferrule dimensions and features are contained in the IEC 61754 series of fibre optic interface standards. One grade of reference connector is defined in this document. The reference connector is terminated to restricted IEC 60793-2-10 A1-OM2b to A1-OM5b fibre at 850 nm band only. The geometrical dimensions and tolerances of the specified reference connector have been developed primarily to limit the variation in measured attenuation between multiple sets of two reference connectors, and therefore to limit the variation in measured attenuation between randomly chosen reference connectors when mated with connectors in the field or factory.