<p>From <a href="http://www.minorplanetcenter.net/iau/info/ObservatoryCodes.html">http://www.minorplanetcenter.net/iau/info/ObservatoryCodes.html</a>: "Observatory codes are assigned to observatories that report astrometric observations of minor planets, comets or irregular natural satellites. The Minor Planet Center's listing of observatory codes is therefore not a complete listing of observatories worldwide." So it seems that these are the MPC's own codes, since the IAU writ large wouldn't need to limit itself this way. Attaching the namespace to MPC allows for potential other observatory codes by the IAU itself or other sub-organizations, so I'd go with /authority/iau/mpc/observatory.</p>