From the website:
The Land Mattress rocket launcher system was developed from the Royal Navy Mattress project. It didn't take long for the Canadian Royal Artillery to adopt them by converting otherwise idle light anti-aircraft batteries into rocket launcher units.
The first Land Mattresses were used in the final days of reducing the Breskens Pocket. They proved so successful that the First Canadian Army expanded the launchers and officially named the unit the 1st Canadian Rocket Battery in February 1945.