Just as an aside, to me it's interesting how horn buttons appear on tweed kilt jackets in all of my vintage Highland Dress catalogues (going back to the 1920s) yet in The Highlanders of Scotland (1860s) all the tweed kilt jackets appear to have ordinary buttons of the period (glossy plain black, looking exactly like the plastic suit buttons of today).

Yes the buttons are very small, but the paintings have an amazing level of detail, and none of the buttons appear to be the type of horn buttons we're used to nowadays. One might say that the artist ignored the actual buttons he saw, and substituted the familiar buttons of the period, but this cannot be the case: the HOS paintings display a variety of interesting buttons (just not on the tweed jackets).