Brass knuckle are all but completed outlawed now in the United Kingdom. There is currently, at the time of writing only two clauses for being allowed to own these;
I do, and have made knuckle by request of a number of studios in London. Often the aquisition of real or prop items for TV is very short notice, time from scripting to production can easily be one week. It is therefore critical to either have these to hand, or be able to produce then in 24-48hrs.
These days with the current restrictions these are not readily available at this sort of notice period from many places, and if someone has never made these, either props or live examples, its highly unlikey they could in 24-48hrs.
I have, and continue to make different casting examples replicated from historic pieces from different period in history. Some more common, to the downright rare and obscure. I like the rare and obscure, which is helpful when I get approached for an example for an actors character who is an arms dealer of very eclectic taste. A 5mm thin piece of curio junk from the continent simple just won't do.
As a footnote; I hate to hear of people who have had historic or period knuckles inherited or in their posession for decades dispose of them for all eternity, cut them up, or throw them into canal's because they're concerned they'll be arrested. I have proof I have supplied items to TV and film for various productions, do reach out if you are wanting rid.
I have only seen a could examples of these. Top two are originals, below is the version I made based on the pictures, made not in brass but in aluminium.
This on is on display in the Musee de la Prefecture de Police in Paris, France. I have not personally seen any other three fingers. This can onnly be used in the traditional 'duster' strike method, it is highly uncomfortable in the straight punch position.
A British design from around the 1940's as I can best place. This perticular model is famously associated with Ron Kray, an East End orgianised crime figure in the 1950's and 60's