Oh what an interesting find.
Just makes me wonder.

That's a 328 grain bullet for a 12b rifle over 4-5/8 Drams of powder.
Hot charge and light bullet.

12b pure lead roundball will weigh 583 grains (~1-1/4oz), and with bullet instead of roundball in the Victorian rifles, I've only seen heavier than roundball not lighter than roundball bullet weights.

How long are those cartridges?
Please will you describe the nature of the source of these cartridges?

--Tinker