W.J. Jeffery & Co was a London gun and rifle maker.
W.J. Jeffery & Co
.400 Jeffery Nitro Express
The .400 Jeffery Nitro Express or .450/400 Nitro Express 3-inch is a medium bore, bottlenecked, Nitro Express cartridge designed by W.J. Jeffery & Co in 1902, intended for use in single shot and double rifles.
Illustration from the 1938-39 Westley Richards brochure
Image: 400 Jeffery Nitro Express dimensions