The MKI liner was designed as a superior replacement to the original WW1 MKI helmet liners (these were uncomfortable, unstable and expensive) as part of the MKI* helmet upgrade programme. Helmets Ltd. designed the MKI liner in 1936, with production commencing in 1937.
The five liner tongues are made of leather cloth (sometimes called rexene) which contains linseed oil. This is then attached to a vulcanised fibre headband, which is then riveted to 4 fibre strips connecting the crown pad - this pad is sponge rubber (sorbo) covered with more leather cloth and riveted on to the 4 fibre strips using 4 rivets. Around the outside of the headband are pieces of folded sorbo that form the buffer cushions between the liner and the helmet shell.
MKI liners are only found in MKI* and early MKII helmets.