That's in fact the gearbox version as it's defined by Voith:
The D851(.1): Retarder only down to 2nd gear, no automatic neutral on stop (up to ca. 1984)
D851.2: Retarder up to standstill, automatic neutral on stop (up to ca. 1994)
D851.3: Retarder only down to 2nd gear, automatic neutral on stop. (1995 and on, the .3E and .5 have the same characteristics, so these must get a "3" for gearbox version, too)