I think you might find that it's a typo error and it is a .411" bullet.

The 465 uses a .468" diameter bullet - the same as the 500/465.

Any idea where I can find the price of Ammo?
Call Holland & Holland as I think this is the only place it can be purchased.

As to why ?, why not, the English gun trade has often worked on proprietary cartridges
and with the type of customers Holland's have, price doesn't matter.

500 Nitro