Avengers, The (1963 series) #3

First Thor vs Hulk battle (an expanded version of which appears in Journey Into Mystery #112).

The Official Index lists the last appearance of the Avengers characters before this as Tales of Suspense #49 where Prof X asks them to help with Angel-turned-evil. Iron Man is the only one to respond as the others are busy in their civilian identities. That issue showed Bruce Banner as one of the non-respondees, despite the fact that Hulk had quit before Iron Man upgraded to his more form-fitting armour in ToS#48. The Avengers obviously must have some way of sending messages to their members without knowing each other's secret identities. I could suggest that they hadn't yet taken Hulk off their phone list. More significantly Hulk is shown as Banner, so he must have used the machine in his secret base to transform between this issue and last. But this issue indicates that, although he still needs the gamma ray machine to become Banner, the transformations to Hulk are now out of his control. And at the end of this issue stress reverts him to human. Iron Man contacts Marvel's non-Avenger heroes Fantastic Four, X-Men and Spider-Man to see if they can help find Hulk (why he thinks Spidey would know I can't guess). He presumably doesn't know about Dr Strange yet, and Daredevil is a few months away. The X-Men are glad to reciprocate following Iron Man's aid in ToS#49. Sub-Mariner is still primarily a villain - in his last appearance in FF Annual #1 he led the Atlanteans to invade the surface world. But next issue he will help bring Captain America into the modern world. And Subby and Hulk will have an entangled future as outsider heroes in the Defenders. Hulk keeps his face on the masthead through to #6, even though he's only in #5 after this.