I've only ever seen a picture of these monstrosities. To be honest, they're not military or military contract as we'd still see reference to them in the old literature. If you want my opinion, then I'd say that they were a cheap (?) method of upgrading the old No1 sighting system to No8 spec. I'd be thinking of Territorial and Cadet Force use in the same way as the carrying handles used to replicate Brens.

I expect that in an old PH catalogue, these things will be advertised somewhere. But definately not Military issue but maybe what we call LPO that stands for Local Purchase something or other using Corps or Unit funds