In 1915 the British army decided it needed what it termed as shrapnel helmets for the troops on the western front, the first regulation helmets were delivered in late 1915 designated "Brodies Steel Helmet War Office Pattern" Designed by John Leopold Brodie, The War Office Pattern is Rimless , This pattern was changed in April 1916 by the addition of a rim and new pattern liner and Chin strap. It was designated as Helmet Steel Mk.I in September 1916. The liner is secured to the helmet by a copper rivet.[see third picture] The rubber ring was added to the liner in 1917 note how the leather chin strap threads through the chin strap lugs and is riveted to the crown. Mk.1's carried on in service until the mid to late 30's when many of them were refurbished with new liners and chin straps and chin strap lugs the helmet was then re designated Mk.1* see the next helmet below.

In the mid 1930s new liners and chin straps were developed and as the old ones became unserviceable on the Mk.1 Brodie's and the first rimless War Office Pattern, they were refurbished with Mk.1 liners and a Mk.1 chin strap, since they were a modification of a existing pattern they were designated MK.1*. The liner is attached to the helmet with a nut and bolt instead of the copper rivet as on the original Brodie.
The MK.1 chin strap comes in two parts and consisted of a 216mm section of 25.5mm wide drab webbing connected by a brass loop to two non-magnetic steel springs encased in drill cloth, the other half of the chin strap is shorter and has a brass buckle, this strap also has two steel springs, the end of each strap has a brass wire loop at the ends. a non magnetic stainless steel chin strap securing lug was introduced for attaching the MK.1 chin strap to the MK.1* helmet this lug replaced the old tin plate securing lugs.
I have come across MK.1* refurbished later in WW11 with MK.111 chin strap lugs and MK.11 chin straps the only way to really know that they were originally WW1 shells is by the makers marks.
The only difference between the MK.1 and MK.11 chin strap is the MK.1 has larger end loops that attach to the chin strap lugs [see MK.11 helmets for a MK.11chin strap]

MK.1* with a MK.1 chin strap, MK.1 chin strap lugs and MK.1 liner.
It was originally a rimless War office pattern Helmet the makers or suppliers mark is H/S for Hadfields Ltd.
The liner was made by Helmets Ltd dated 1937 it also still has quite a lot of its sand paint left. On the side although faded is a flash for the Royal Army Service Corps blue, yellow, white.

MK.1*.Originally a MK.1 Brodie, the makers mark is unreadable the MK.1liner was made by Vero dated 1938 Unfortunately all that remains of the MK.1 chin strap on this helmet are the end brass loops.
With this helmet being red I am thinking it was perhaps used on a airfield by the RAF as a fireman's helmet in WW11?.