Sgian Achlais How was it secured under the armpit?

I was thinking the same thing...

Thank you, Keith. I wasn't even aware that these knives existed. 
I found a site, xmarksthescot.com that comments on shirts having slim pockets on the upper sleeve to hold the knife. But there is also a picture of a knife with a sheath that has a clip (similar to those on Highland pistols), and it would fit over the waistcoat at the armpit.
www.xmarksthescot.com/forum/f263/sgian-weapon-fighting-61355/

Richard/Grumpa

Probably a button hole in a waistcoat near the breast or under the armscye where the frog could attach.

Keith,

Thank you for sharing the information from your inquiry.

The below link shows the same knife along with three other period Scottish Knives.  Does the Gralloching (Hunting) Knife on the left look familiar?

i.pinimg.com/originals/fb/3e/f9/fb3ef9333e4964175c60d8727b37091d.jpg

Please notice the "jimping" or notches filed into the top of three of the blades.  This decoration was also known on some period Scottish Dirks.  

Also, the use of Red Stag handles seems a bit more common in Scotland for knives, generally, and even for some small utilitarian knives.

Gus

Hi Blackhand,

Yes they are, though in an alternating pattern.  IOW, when you see two notches on one side, there will be a notch between them on the other side - and it goes that way down the length of the jimping.

Gus