Normally, A10 taps were the same on both sides. 'Reserve' was achieved by the shape of the petrol tank. If your tank is from the standard model it should have a raised floor on the right hand side (the side where your right knee is). This arrangement causes the right hand side to empty first. So, rider's instructions are to only use the right hand tap, when running, and open the left hand tap if 'reserve' is required. Your tap is a later type, supplied with the optional standpipe for 'main', or without a standpipe for 'reserve'. On the A10, the standpipe is unnecessary for the reasons I describe. However, if you do fit a tap with a standpipe, you must fit it on the right hand side and you will need to go onto 'reserve' much earlier than is desirable. You'll probably end up with about 100 miles worth of reserve!