I hope the owner gets to shoot this old warrior again, most probably a tricky situation if based in India - one can only hope so!
Magnificent rifle, I hope he gets to pass on this worthy heirloom!
Bertram Brass do make them
https://bertrambrass.com/cartridge-list
https://us.reloading.co.uk/bertram-brass-577-500-3-1-8-formed-20-pack-bm990.html