No. The standard is case-sensitive. You will often see it written as (lowercase m, uppercase K) or Mk (rare). Technically, "M" implies the linear class, "K" implies the geometry class. Writing "MK" in all caps is common in file naming but does not change the numerical values.
This part applies to external and internal sizes, diameters, radii, and distances. The permissible deviation (±) depends on the nominal length of the feature: Nominal Length (mm) Medium (m) Class (± mm) over 3 to 6 over 6 to 30 over 30 to 120 over 120 to 400 over 400 to 1000 Data source: ZEISS Quality Forum and Eurotools . 3. ISO 2768-2: Geometrical Tolerances ("K") Tolerance Iso 2768 Mk Pdf
Permissible deviations for straightness and flatness depend on the nominal length of the feature being measured. Technically, "M" implies the linear class, "K" implies