Sub-Mariner #34.  This book features what is referred to as the "Defenders Prelude."  Defenders #1 was released almost a year and a half after this book came out.  It's when the Silver Surfer, Hulk, and Namor the Sub-Mariner get together as a team for the first time.  Sal Buscema draws a beautiful cover and also does the interior art.  Story presented by Roy Thomas.  This Bronze Age key book is graded 9.2 NM- with off-white to white pages by CGC.