http://www.velovision.com/showStory.php?storynum=946

They seem to think the hub is 940g without sprockets. That sounds about right and is significantly lighter than any other SA 3 speed hub. So with 120g difference between it and the S-RF5 you have to then add extra sprocket, cable, jockey wheel pusher mechanism and shifter, which according to the brompton website is around 180g (that's the difference between 3 and 6 speed weights. Which means something like a 60g difference overall.

The old Sprinter hubs had steel shells so they'd be 100g heavier than the current type, in that instance they'd be heavier overall.