I'm sorry if the term is not right, but I've always known these as stripper clips, I had no idea it wasn't the correct term. But even if it isn't, I guess it is still pretty common as this is the definition I get from a quick search:
''A stripper clip (also known as a charger or charger clip, especially in British and in Commonwealth military vocabulary) is a speedloader that holds several cartridges (usually consisting between 3 to 10 rounds) together in a single unit for easier loading of a firearm's magazine''