This is we believe what's known as a "drift sight" that once belonged to a British Lancaster Heavy Bomber. This item came from Australia some years ago. It's designed to allow the bombadier to account for wind-drift before dropping a payload over target. Overall, it's in fair condition with a couple of pieces missing which include the rubber piece that fits around the lens and one of the dials. The missing dial was probably gov't-removed before being sold off as surplus decades ago. A rare and historic World War 2 British aviation item.