British Drill Trousers (AKA Safari Trousers) were limited surplus items from 1980 until Fall 1983 when they were mass-produced by BR through 1985. They have the same waist as the Gurkha Shorts.
From the 1980 Catalogue No. 4: “These spectacular British safari trousers, an inspired creation of Her Majesty’s tailors, were made for colonial wear in the early 1950s. As the Empire crumbled, the pants piled up (never worn) in a London warehouse, which is where we found them. This is not only an authentic collector’s item, it’s the best pair of trousers you’l find at any price. We live in them. The only remaining sizes will for men from 32-inch to 37-inch waists. All others: sorry, you’re too late.”
From the 1985 Catalogue: “An inspiration form the British Empire’s masterful tailors, these drill trousers were designed as an extremely comfortable pair of pants that would allow maximum freedom of movement. We’ve matched the original drill cloth (100% cotton, of course) and recut the pattern into a more contemporary fit. Dashing on men and quite flattering to women.”