The BMW R12 petcock or fuel valve is a 3 position petcock. Straight down is off (Zu) which is labeled as so. Than if you turn it to the right it is (Auf) and this is on. Than by turning the valve to the left is Reserve. The bowel has a fitting on the bottom which allows you to remove the bowl and change/clean the filter. For the outlet it is one line for the SUM carb ( vergaser). If you have the sport model R12 with dual carbs you would have a dual outlet, one line leading to each carb.
From BMW high res factory photos the petcock was cadmium plated when delivered from the factory.