I have known of the “Sgian Achlais” as the predecessor of the Sgian Dubh for a long time. My understanding was that the old story of “when you go into a friend’s home, you put your Sgian into your hosetop so you’re not concealing your weapon” applied directly to the Sgian Achlais. IE, the first Sgian Dubhn were simply Sgian Achlais stuck in the hosetop. And they evolved to be a shorter shape from the longer skinnier shape to become the modern Sgian Dubh shape.

I have no sources, but that has long been my understanding of the origins of the Sgian Achlais and Sgian Dubh.