Just to add to the confusion, here's the text from the original Hurth manual for our gearbox (pre ZF) HBW 150 :
CAUTION. Idling position of the propeller: gear shift lever must be in «O» position. Use the shift position opposite to the direction of travel for locking the propeller shaft, otherwise the transmission will be damaged.
...I guess the next time we're sailing backwards I'll have to remember to shift to "F"