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There's a suburb of London called Cockfosters, which is thought to have been connected to/the residence of the Cock (or head, or senior) Forester. Which makes your sighting look geographical (and badly written!).

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I would doubt the -foster = foster child equation, simply because surnames came into use long before fostering was institutionalised. Back then, if you looked after a child it was done informally, out of the goodness of your heart or your wish for a cheap labour force. And now, the child's ln would never get a marker like that.