Apart from the obvious Sterling pistol grip, nobody at Sterling seems to know anything about that rifle, not least, David Howroyd. And you'd think that he would probably know - him being the engineering manager and then director of the company.
As for the Q in post 2. Ord were destroying large amounts of worn out Sterling SMG's in the late 70's and 8o's and as Sterling had always had good relations with Ordnance, supplying spares and tech info regarding the use of the ex India order guns broken up for spares, they asked if they could buy back the remaining 6,000 or so ex India Mk4 KR marked casings. They did - and these brand new KR marked/numbered casings were made up into complete guns and sold on the market along with the current 'S' marked guns. And a healthy profit was made as they'd already been paid for once so all Sterling had to do was to assemble new parts into them.
At the same time, Sterling asked to purchase the ex military guns that had been reduced to scrap by welding (see photo in book) or guillotining. This was agreed and as 'scrap carriers' Sterling were required to become industrial scrap merchants!!!!!!! These old casings were totally unpicked of thousands of valuable parts by the work force during slack periods and even better, on a bit of overtime pocket money on Saturday mornings. Many tons of parts including 'welded detail' trigger frames, mag housings, front and rearsight protectors, latch brackets, finger guards etc etc were unpicked, tumbled and re-used on new production guns. KR, US, UF (not the flanged type) marked mag housings were sent out on new made or FTR guns to other nations.
It is for this reason that you will occasionally see F/Fazakerley 'welded detail' marked parts on Sterling made guns. Backsight ears and foresight protectors are the most common.
I have been asked to elaborate on this in more detail for the new edition of the book