The .400/.375 Belted Nitro Express aka .375 2 ½” H&H aka .375 Holland Velopex are all the same cartridge, different loads and names only, introduced by H&H in 1905. H&H converted Mannlicher-Schoenauer and Mauser actions with barrels made in England to use this cartridge. Rifles in this chambering were not made by the Steyr factory. H&H most likely ceased making such rifles after they introduced their .375 H&H Magnum in 1912. The proprietary .400/375 was probably already on the way out after Steyr introduced the 9.5x57 M-S aka .375 Rimless Nitro Express aka .375 Westley Richards Accelerated Express aka .375/43/270 WR (proprietary load with the WR Copper Capped Expanding bullet) in 1910. The 9.5x57 as a Steyr factory offering with widely available interchangeable cartridges made the proprietary H&H .400/.375 superfluous. In his book “Mauser- OOSR”, page 238, friend Jon Speed shows a H&H takedown rifle on a Mauser action, chambered in 9.5x57 M-S. As the original case label, “our special solid brass case”, and a chamber cast told us later, it was rechambered from .400/.375 H&H to the more readily available 9.5x57 M-S during the interwar years. The .400/375 had a belt-rim diameter vera close to the standard rimless .473” base, but with a slimmer case body. The “new” 9.5x57 chamber shows an unusually long neck area, left over from the longer .400/375 chamber.