.400 PURDEY LIGHT EXPRESS 3 inch BPE.
The headstamp is Kynoch 400 P which stands for the Purdey again unknown year of manufacture.
Introduced by gunmaker James Purdey during the late 1880's. Cartridges were loaded for Purdey by Kynoch and Eley.
Usual MkVII 303 for scale.
I also found this place as a guide for the N.E's so others may use it if they wish, the .400 P cartridge was not listed in my COTW(14th Ed.) and information on the web very limited.
400 2 KYNOCH | Cartridgecollector.net