There is one of these on the unmentionable auction site y-bae at the moment. I know collectors of BSAs are always looking for things like this so I pass on the information, I don't know anything about the item or the vendor but the picture looks favourable and it is cheap (at the moment).

For those of you under 40 it is a 'skeleton' shoulder stock which can be fitted to the grip of the large, heavy BSA Scorpion pistol to make it a small carbine. The Scorp is from the 'lets produce a pistol - I know lets just chop down one of our rifles to fit' school of thought, also followed by German competitor BSF.