The copper plug is actually a very thin copper tube, fixed in place during the final swaging operation. I have dimensions on them, too.
Tom should be able to make a mold - I used to, but I'm retired now. The 255 grain bullet is the same, but with no hollow and plug. You can fill the hollow with wax, it'll do the same thing.

Have you read "The Modern Sportsman's Gun and Rifle Vol.2" by J.H. Walsh 1883? If not, you need to.

The only guy making correct Express bullets (only for the .450) is Jim Poynor in Virginia. Very deadly with a lung shot on deer, that's for sure.