It was decided in the mid fifties that a new more comfortable liner should be introduced for the mk.1V helmet. It was approved for introduction in March 1959 and designated as the MK.4 liner and subsequentley the complete helmet was designated MK.5. Although there is some debate about this with some collectors saying it is still designated as a MK.1V helmet with a different liner? The MK.5 remained in service until the mid eighties when it began to be replaced by the Plastic MK.6 This helmet has a mk.4 chinstrap same as the one used on the mk.4 helmet