Abstract: An adaptor (1) can be releasably connected to a fixed plug portion (13 or 21). The fixed plug portion is, with that, for example a component in a measuring device for taking measurements on differing optical fibers. The adaptor is provided with a sleeve (3) in which various transferable plug portions can be temporarily inserted. The sleeve has an axially parallel slot (6) for accommodating a rotational security device on an initial category of fixed plug portions (21). Additionally, the sleeve also has at least one further recess (4, 4, 5) on its outer casing for accommodation of the rotational security device (19, 19', 20) of a second category of fixed plug portions (13). In this way, an interface between adaptor (1) and fixed plug portion (13, 21) is created, which indeed enables combination with an adaptor equipped in the same way, although an adaptor without an additional recess is only able to be placed on one of the two categories.