Sorry, this has nothing to do really with Paul's issue, but I did find it interesting that in the old c1940 RG Lawrie Highland Dress catalogue that I recently aquired, underkilts are listed right along with kilts, as if they were a standard part of Highland Dress.

They are listed as

Under Trews all-wool tartan 18/6

and are listed both with Gentlemans' and Boys' Highland Dress.