The following 4 covers exhibit dark marks near/around the stamps - what is that?
Classic "toning" or "Rust" marks, indicates storage in an humid environment, it generally begins at the fine hairs of the perforation teeth.
The first stamp, is mucilage (gum) that has lost its viscocity in an humid environment, then re-hardened / re set in a stamp mount, giving it that flat mirror looking shine.
The rest could be anything, depending on the owner at the time.
The last item would probably be private gum of some sort, as the item is a mutilated stationery piece, affixed.