In The Case of the Black Sheep, Lagganmalloch was an island in the Scottish Hebrides which had been used as a seaplane base during the Second World War. It was later purchased by Flight Lieutenant R. Q. Paullson who used it to raise black St. Kilda sheep. Paullson had registered a de Havilland Gipsy Moth seaplane which he used to communicate with the mainland. Over several occasions, he used the seaplane to retrieve contraband nylons which had been thrown overboard by an accomplice from the ship Sirocco. The parcels of nylons were taken to Lagganmalloch where they were hidden in bales of wool. The wool was then shipped by means of MacRowland's coastal steamer service which called every Wednesday at the island. The cargo was delivered to Baltroonie consigned to Louis Vanberger who masterminded the entire smuggling scheme.
Lagganmalloch is a fictional island and Johns did not specify its location and any other geographical details.