Marrakai Tony has nailed it, the ".400 Purdey Light Express"......emphasis on "Express"
According to the Fleming book, introduced as a Nitro for Black or more correctly Light Cordite cartridge in ~1905

Back in the days leading up to this cartridge, the usual .40's were not considered as a large game calibre. Red Stag, Deer, and in India, Black Buck, Boar, and possibly Tiger from a howdah. Made up into nice slim light weight stalking rifles.
- Mike

Here are a three:
https://www.gunsinternational.com/guns-f...un_id=101031536

https://www.gunsinternational.com/guns-f...un_id=100961270

https://www.guntrader.uk/guns/rifles/pur...200320125326001