"Arthur" as it is known is the first release of the operating system developed by Acorn for their Archimedes line of computers.

Arthur was developed purely in reaction to the fact that Acorn's intended operating system for the Archimedes, ARX, would not be ready in time for the release of the system.
The team working on Arthur was given a mere five months to complete it under the notion "just make it work like the BBC Micro", with the intention of using Arthur as a stop-gap. However, continual delays and the realization that Arthur could accommodate a full desktop environment led to the cancellation of ARX.
Also working in Arthur's favour was that it worked on the first systems with 512KB of RAM and a floppy disk. ARX required 4 MB and a hard drive.

These screenshots were taken using Arculator, an emulator for various models of the Archimedes.

The default Arthur desktop.
The first thing you will notice is the eye-searing default colour scheme, referred to as "Technicolour" by some.
The second thing you will notice is that this is very similar to later versions. Disk icons appear on the left side of the bar, while the bundled accessories appear on the right. There is no Acorn menu, just an exit button.

Here is a look at a disk opened in a window. There are no different folder "views", just the icon view you see here.
Arthur runs in ROM, but only includes the desktop accessories. This disk contains the rest of the operating system.

Here is a window maximized. Maximized windows do not cover up the bottom bar as in later versions.

The button on the right side of the window is a "send back", that of course does as described. There is no minimize function, so window management can become quite a pain if you have several of them open.

   
Here are two types of programs running under Arthur. Pretty much the entirety of Arthur is written in BBC BASIC, although the bundled accessories are written in assembler.
Like other early computers there is no multitasking here. Later internal only releases of Arthur did feature very rudimentary cooperative multitasking support, similar to the Mac OS MultiFinder, and allegedly application-level support for it could be added to the public release.