I have only seen this item manufactured in tinplate. It will fit into the same recess as a standard Porter escape, but it allows a greater flow of bees through it's body as it has eight exits.
Stiffness is achieved by an embossed rim around the top plate.
The valve action is produced by eight thin strips of brass or phosphor bronze (shim material) that are soft soldered to the underside of the box section body.
The various views and cross sections shown left should indicate the construction.
Note that the non ferrous springs are soldered to the body some distance from the edge of the exit hole. this makes the spring more compliant
The view at extreme bottom shows the way that the springs are pushed out of the way by the passage of bees.
I used to sell these as part of my beekeeping supplies business, but I have no personal experience of using them.
I was always of the opinion that the rather flimsy springs would be unreliable in field use and tinplate has a habit of rusting, particularly after soft soldering has been carried out with corrosive fluxes.
I have not seen these items on sale for several years.