I think they were hammered over a buck, the screws timed,and then the plates and screws filed to final form on the rifle. Very possible that they were smoothed out on a small English wheel though too.
Now, how they filed the plates to mate so well with the action and tangs eludes me. Must have been a matter of file - fit, file a little more - fit, file some more...
I would really like to see measurements of the known Gibbs rifles with Selous plates. There is a distinct possibility that the plates were stamped out and then the stocks shaped to fit. I say this simply because all of the Selous plates on Gibbs rifles I have seen have very consistent form and dimensions.
Also, I would like to see a Gibbs with the plates removed. That would tell us a lot.

As for the straps in the case...
I joked that since this rifle was made at the same time as the "Scramble for Africa" it would have been advantageous for an adventurer to have the national flags of Germany, Great Britain and France all stowed away just in case the borders changed or the party inadvertently crossed over to another country's territory while out on safari!
Actually, I am at a loss as to what they are really for.