I don't know a lot about the discharger cups but some years ago, I managed to have a look at a fair few, both in the UKicon & in Franceicon, & most of them had S&B marked adapters, with a variety of different markings (& sometimes very little) on the actual cup itself. I wonder if S&B were a/the major contractors for this component.............or as you suggest, S&B is not a manufacturer's mark after all.

As an aside, do you know how late on RFI made the cups? Did they continue to make them after the end of WW2? Just curious.....