Here's a list of known makers of British WW1/WW2 helmets and Liner Markings.   British Steel Suppliers of WW1 British Helmets.   V = Vickers Ltd of Sheffield.......................1916 - 1917. MS= Miris Steel Co Ltd of London..............1916 - 1917. ( also produced helmets after Aug 1916 ). also used "MLS" marks. FS= Thomas Firth & Sons Ltd of Sheffield............1916 - 1918. (  also used "F" mark and most likely used "FKS" marks ).. O = Samuel  Osborne