This is from the early days of modern operational security. This would be far more difficult today. If only the bomb makers had talked to the ketchup people at Heinz. Rumor has it that Heinz regularly purchases extra ingredients, disposing of the extra bits on site, so that nobody can infer their secret recipes by counting what goes into the factory. Given the security budget for protecting nukes, it should have been simple to double-order both lithium and thorium.