Back to my Allen Brothers theory, it's interesting that they drew and painted themselves wearing what appear to be moccasin-type Ghillie things, the same sorts of things they show in their fantasy illustrations in Costumes of the Clans.

These appear to have been picked up by McIan, who shows them in a number of his fanciful illustrations of "ancient" Highlanders.

However in these photographs it can be seen that both John and Charles wore Ghillies with soles and heels.