I have found the difference in the 1st (1798) and 5th (1822) versions of the "Elucidation". The latter has an extra section on pages 28-31 covering Movement by Ranks in Threes. It is otherwise identical.

This drill for moving in threes was certainly in use during the Napoleonic wars since several memoirs refer to it, notably Tomkinson's "The Diary of a Cavalry Officer" which is packed full of detailed descriptions of tactics.

Rod