Nifty "snail" bullet trap there.

Love the price of the cartridges: 50 shillings (4.5 real pounds, Sterling) per THOUSAND.

Then again, in those days, that was probably close to a week's wages for the basic punter.

It can't exactly be rocket surgery to whack a piece of 4140 in the old lather and turn up something similar to use .32 S&W / .32 Colt, etc. if one were so inclined. The more adventurous (and well-heeled) could try 7.62 Nagant.
Did these "special" cartridges have a slightly large diameter rim so that the .303 extractor would engage the case properly and actually "extract", leaving the adapter in place?

The thing about the good-old rimmed .303 is that you can use, and re-use, the full-sized case for "gallery" loads as long as you take care and use a "bulk" powder that takes up a LOT of space for weight. If you are running a Mk1 Metford, you could even use REAL Black Powder or Pyrodex for some extra "authenticity". A "heeled" bullet as found in some of the "Rook" cartridges and having a max diameter of about .317 may be useful as well.

Dust off the pith helmet, ammunition boots and red tunic and have at it, as they say in the classics.