The Concorde stamp has two vertical phosphor lines running around 5-7 mm into the stamp. If you angle it to the light they will show up as dull bands compared to the rest of the stamp, assuming you do not have an UV light.
There is not a watermark.
This stamp does come with missing phosphor and missing colours but basic stamp is of minimum value and as yours is damaged it is needing replacing.
It is fluid safe if you are using plain water, no need to use anything else as it had water soluble gum.
As to the USA stamp someone else will have to comment as I have no knowledge but as you say badly damaged.

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