That's a very interesting cover.
This was sent to Viscount Melgund
, probably from a family member at the Minto estate (as identified on the back).
Viscount Melgund is a title held by the Earl of Minto, and this would normally be used as a courtesy title by Lord Minto's eldest son.
In 1886 Viscount Melgund would have the future 4th Earl and future Viceroy of India
and later Governor-General of Canada:Gilbert John Elliot-Murray-Kynynmound, 4th Earl of MintoMinto
is a small village close to Hawick in Roxburghshire (my home county).
I believe the address on the back has the old-fashioned abbreviation, N.B. meaning Scotland, i.e. North Britain.
I agree this is a mourning cover.