The BMW R12 in the early stages of development did not show the Motor guard in the parts book. It is unclear the exact date of production but we do know it appeared in 1941 parts book. In 1941 the motor guard was listed as accessory F66U 195b on page 93 parts book D605/4.
Below is an original motor guard with the part # on it. Note that this number is different than the 1941 parts book D605/4