curious about a MK I sight variation

I am curious if a MK I sight variation I have seen is a real item or something made up in the UK gun trade.

I have seen this sight 3 times now.

once on a ex-cadet No 4, once on a Sterling like No 4 conversion and it is shown on page 263 of Skennerton's "the Lee Enfield" 2007. On page 263 the sigh is shown in a MOD picture of No4 target rifles at the bottom of the page, top rifle of the three. The top has this sight, the second has a 8/53 and the third has a late 5C on what appears to be a Parker Hale T4 rifle.

The sight seems to be nothing more than a MK I No4 sight with a small aperture mounted in in, it is non adjustable but gives a better sight picture. Does anyone know the story of these sights?

If it's the sight that I am thinking about, it is a commercial update of the Mk1 No4 sight. Not UK military issue except that a mickey mouse variant was fitted to the P-H L81 rifle, which used the No4 main leaf or the No5 leaf.