Afterthought: Westley Richards criticized Holland&Holland's .400/.375 Velopex load in their 1912 catalog, page 111. After describing the ballistics of the .375 rimless Nitro Express aka 9.5x57 M-Sch, 270grs at 2200fps, they warned:
"N.B.-The weight of the bullet with which the 2,200 ft.sec. velocity is obtained should be noted.
The velocity of 2,200 ft. is the highest obtainable with the 270 grs. weight of bullet in the .375 rifle; higher velocities, which are obtained by using lighter bullets, increase the penetration at the expense of expansion and general effectiveness, and are not to be recommended for sport." End of quote.
Though obviously partly wrong, this is apparently directed against Holland & Holland's .400/.375 Velopex load, as advertized in H&H's 1910 catalog, page 52 http://www.rbsiii.com/nitro_express/H&H_1910_Catalog.pdf
The patented H&H Velopex bullets were very light for the calibers, the jackets partly filled with some fiber instead of lead. H&H don't mention a bullet weight. They only mention a then staggering mv of 2500 fps and a striking energy of 2583 Ft. Lbs. Trying to calculate back from these numbers, I land at a bullet weight under 200 grs. Apparently this fad for ultra-light bullets did not last long.
The post-1928 ICI catalog lists the .400/.375 NE (Holland) with a 270grs bullet, 43 grs Cordite for 2175 fps from a 28" barrel, about the same as the other two non-Magnum .375s.
The modern Kynoch load is advertized with a 235 grs bullet at 2400 fps:
http://new-kynoch.apt-sites.com/400-375%20belted.htm
Why should Westley Richards pre-WW1 have chambered a rifle, except perhaps on very special order, one of their rifles for a cartridge they did not like at all, and only available from their competitor??