This part of the IEC 63267 series defines the dimensional limits of an optical interface for reference connectors with 2,5 mm or 1,25 mm diameter cylindrical zirconia (ZrO2) ferrules necessary to meet specific requirements for fibre-to-fibre interconnection of non-angled polished multimode reference connections as defined in IEC 63267-2-2. Ferrule dimensions and features are contained in the IEC 61754 series of fibre optic connector interface standards. One grade of reference connector, Rm1, is defined in this document. The reference connector is terminated to restricted IEC 60793-2-10 using A1-OM2b to A1-OM5b fibre in 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 reference connectors, and therefore to reduce the variation in measured attenuation between an arbitrarily chosen reference connector when mated with a connector in the field or factory.