https://www.milsurps.com/images/impo...scan0001-1.jpg
Here is a photo that would seem to have been taken on the same occasion as the photo on the dust jacket of Peter Laidler's excellent "The Sten Machine Carbine". The other gentleman in the photo is Mr. Thompson, Churchill's bodyguard.
It is hard to see in the scan, but the rear sight on this gun is non-standard. It appears to be made with a curved saddle which partially wraps around the head collar. In the dust jacket photo, you can just see that the gun has a sling swivel fitted to the front of the barrel nut. The gun was no doubt a prototype of the Mk. II, and these features were not incorporated in production guns.