I found this awesome photo of Boeing's 367-80 "Dash 80" prototype for the 707. It appears that they have mounted a 5th engine on the tail to test out some systems for the 727. I'm wondering though: what engine type is that? Is that one of the JT3Cs used on the Dash 80, or one of the JT8Ds for the 727? It looks a little small to be a JT8D
