The numbers inside the bullets are new to me, or my memory fails, either way I am sure there is something vaguely important there… British Bullet Guys???
Originally, the Minié Balle [bullet], as designed my Captain Claude-Etienne Minié and Henri-Gustave Delvigne, an armourer, in 1847 [+/- one year?], had an iron cup in the base to force the bullet to expand against the rifling, sealing the gasses behind for greater speed and accuracy at greater ranges.
Some countries used cork, wood, and a host of other things to do this job.
I have seen pictures of bullets that had some sort of fabric in the cavity ahead of a wooden plug, I suppose it was intended to stop the forward movement of the plug.
The body of the bullet is generlly called the “skirt”, at least among those I used to shoot against, competatively, that is.
EDIT to add:
I should mention that this is my well-after-midnight-overly-tired very brief explanation of how the Minié Balle works, and why it works so well.