Inside the standard R20 stem there is an arrow-shaped slot ground into the stem as well as a strange wire contraption that somehow helps to keep it all together when you tighten the quick-release clamp. (It's a while since I dismantled mine
)
You can definitely fit any 1" quill type stem in the headset but I don't think you can also take advantage of the clamp and turn the bars 90° on the fly when you want a quick fold. I may be mistaken but I'm pretty sure the bespoke nature of the Raleigh stem is entirely to do with this 90° swivel fold and don't think normal quills will work the same way.