Shouldn’t be too difficult.
Who exactly are being appealed here to keep hold their tongues and keep the secret a secret? A two-word phrase is the key, and the muscular gentleman is a huge giveaway.
Happy quizzing, and have a nice life, folks!
Four answers, and all correct. This was indeed at Oak Ridge in Nashville, Tennessee, in the US, reminding US government employees working on the Manhattan Project not to be the cause of a leak of information on the project.