With thanks to Bob Bennett
Lancers were introduced when the 9th, 12th, 16th and 23rd Light Dragoons were converted in 1816. It took some years to settle, but eventually the crimson / gold (officers) and scarlet / yellow (O.R.s) became the norm. Girdles in these colours thereafter became standard wear for all the Lancer regiments, regardless of their individual facing colours.
Approved 19 May 1875, List of Changes (LoC) Paragraph 3599 introduced a modified Girdle, now reinforced on its inside face with leather. Unfortunately, LoCs date only from 1860, so there is no official documentation yet found, which introduced the original girdle - though this is probably over a century prior to 1905, as the Lancer Girdle was apparently the Light Dragoon Girdle being perpetuated. Those Light Dragoons who later converted to Hussars, standardised on the Barrelled Sash instead.
The reinforced Girdle, worsted, Lancers' was to enable the newly introduced Case, leather, brown, B.L. revolver to be worn on the Girdle, the Army finally having decided to take a revolver into service.
With thanks to Rog Dennis