The rear flap is for the internal water tank.
When I got my Westy there was no key. I took the complete assembly to a
locksmith and he made me two keys. the ones that he made are small metal keys
looking like a small padlock key.
You can get the complete assembly with keys for about $50.

Sam W
63 singlecab
70 bay Westy
87 Vanagon Westy