Yep, I think of Arthur as a sortof Beta for RiscOS

I had an Archimedes A305 which launched with Arthur, when I upgraded to Risc OS (which arrived on ROMs, if I remember correctly, you ordered the ROMs and then your local dealer would fit them onto the motherboard) it wasn't really _that_ different, everything I'd written continued to work, etc. Like it was a major upgrade but fundamentally the same "thing".