"6VS" is what the wire is labeled, not necessarily what voltage will be on it. The designer of the system might just as well labeled the wire "foobar".
I agree that 7.4 V is probably fine. What is odd is that the two windings on the transformer are used in phase, only giving half-wave rectification. The rectifier is a double diode, so it would have been just about as easy to have full-wave rectification.