M-B's internal engine code for the Mitsu 3B21 is "M132." "M" = "Motor;" "1" = a gasser. The Suprex gasser is M160; a Suprex diesel is OM660. OM = "Ölmotor" (oil motor), and all passenger car diesel codes begin with "6."
The logic of "32" falls in with the previous 30s codes for the Mitsu/DC engines for the smart forfour: M134 (4A90, 1.3L gasser) and M135 (4A91, 1.5L gasser); and OM639, a 1.5L M-B diesel triple derived from the "A Class" four. The first forfour had a MDC 3A91 1.1L gasser designated M122.