Check out the Vintage Airstream Photo Archives.
This is a great resource for checking out the features of the various models.
The photo looks more like a water heater access panel, or what is left of it, than a furnace vent. These are hinged at the bottom and held in place by a cam-loc at the top.
The Archives show the outside access panels you asked about in the other post. The doors for outside storage lockers were simple double sided panels wit a lip that fit in a groove at the top and there held in place at the bottom with a tumbler spade lock.
Hope this helps,