A single sporran for very casual to semi-formal?

[MacMillan of Rathdown]

While military regulations may specify a black bow tie with mess dress when attending a white tie event, civilians should follow the conventions of wearing a white tie (or, depending upon the style of the jacket, a lace jabot) when in Highland attire. The idea that black tie could be worn in lieu of white tie dates from the end of WWII when His Majesty, King George VI, for a brief time relaxed the white tie requirement at dinners when members of the royal family were present. This was done so that those not possessing formal evening attire due to war-time and post-war shortages could still attend.