In the Cal Pappas 2008 book ".600 Nitro Express" he identified 204 .600 N.E. rifles, a mix of Doubles & Singles (pages 182 to 193) made between 1900 and 2008. W.J.Jeffery made the most with 70 (37 Double Rifles & 33 Single Rifles between 1900 and 1929) and then Westley Richards with 28 and then Heym with 17 and Holland & Holland with 16.
Of the 204 .600 N.E. rifles; there were 104 made up to 1939 & 91 after 1939 to 2008 (with 9 Unknown dates).