Trying to look for it. Could it be that this is some limitation because I am playing a free Itch.io version or something? I mean, I do see the lever "in the air lock door itself". But it isn't a separate object but just part of the door. I rechecked the room and I truly think that, at moment in the game at least, there is nothing else to be interacted with in the room besides: two open doorways, air lock door, and mailbox. The key hook/chain thing isn't an iteractable thing either.
But, thanks for the help anyways :-)