from what I know there are several brands that are similar. They do hold as tight as drilled and tapped. They are not custom made for individual rifles so with the m slight difference in dimensions of each rifle and mount there is some play, however small.

I believe you can get them for the No1 Mk3 and the No4/5. I am about to start looking for one for the No4/5 as i have trouble seeing the front sight clearly on my Jungle Carbine. Only use it in scrub after pigs so any slop wont really matter.