I've just been looking at the beer can collector's site www.cannyscot.com (geddit? geddit? You see, the site is about Scottish beer cans, and, oh never mind), well worth a look, and this press cutting from 1956 caught my eye:
Tennent's also have a large stout trade. In export markets the company features three stouts to suit the palate of customers in different parts of the globe—XXX stout, dry stout and milk stout. In the home trade their sweet stout is one of the most popular bottled products of the day.
One bit of inconclusive, circumstantial evidence. I wonder if this terminology was actually used in the trade back when breweries made several different kinds of stout? They must have had the same problem of differentiation that Michael Jackson faced.