I believe ever started with EB mods and now AMR but more manufacturers have these from own production like REDS and MAX, I do not know how far back but I have heard stories of people with Serpent Impacts running these backplates back in the 90's.

I have made some before for OS, Novarossi and a Picco boost and now I was asked to make 2 for a new Picco Monza. The dimensions given were exactly the same as de one of my Sonic/Picco PSS so I had the right example at home.
The plate is exactly the same as I have made before but with one difference, no flat spot for the piston but a groove. It was on my list for a next one so I made a little helping tool for that.

Instead of pictures and tekst, now a short movie: