I've been reading through another Osprey publication; not entirely for kilt related reasons, but the kilts and Highlanders are in there.
Tomahawk and Musket-French and Indian Raids in the Ohio Valley, by Rene Chartrand and Peter Dennis (Osprey, 2012).

In one place it says some of the Highland regiments started out with jackets that had no lapels; later being ordered to switch to jackets with lapels. That would be under General Forbes.

How exactly did the jackets not have lapels? It doesn't seem to explain.