This is a genuine Leitz (Leica) ISOOZ / 14099 13.5cm (135mm) LTM-to-M mount adapter ring. It is the official factory accessory made by Ernst Leitz GmbH Wetzlar, Germany (post-war “DBP” = Deutsches Bundespatent marking). Its purpose is identical to the 9cm version you showed earlier: it lets any Leica screw-mount (LTM / M39) 135mm / 13.5cm lens (most commonly the Hektor 13.5cm f/4.5 or Elmar 13.5cm f/4) mount perfectly onto a Leica M-series camera body (M3, M2, M4, M5, M6, etc.).

This is the single-focal-length version of the adapter (earlier style, typical of the M3 era). Later combined adapters were engraved “M2 21-35 M3 135” or similar, but the function is the same: it selects the proper viewfinder frame lines automatically when you mount the adapter + lens.