STEN is an acronym, from the names of the weapon’s chief designers, Major Reginald V. Shepherd and Harold Turpin, and EN for Enfield.
The Sten gun was a family of British submachine guns chambered in 9mm and used extensively by British Military throughout World War II and Korean. The guns are simple in design and are cheap to make so they were made in high numbers.
The Sten MKIII was entirely encased in a wooden body (the featured image is not a MkIII), with the only external metal parts being the trigger, barrel, magazine and cocking handle. The trigger and pistol grip were in line with the magazine. The reasons for its creation are likely an experiment into increasing the comfort and handling of the weapon in freezing temperatures.
I have a parts kit for a Sten, and I think they are pretty cool guns to mess around with, but I am not a big fan of fully automatic fire for serious work, most people lack the experience and skill necessary to keep a fully automatic gun on target and I believe that if it needs shooting you need to hit it not around it.
But yes, it the current laws ever change, I’m getting a piece of fence pipe and turning my Sten kit into a Sten gun. I just have to wait until its legal.