There is probably a term for this addition, but I don't know what it is. When we add something like this, we are simply clarifying, or modifying, what we've already stated. Not back then serves as an adverbial phrase telling when I couldn't imagine something.
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PhilipNot back then serves as an adverbial phrase telling when I couldn't imagine somethingOr when he couldn't fight like that; it is not clear to me which is meant.
Mister MicawberOr when he couldn't fight like that; it is not clear to me which is meant.Good point!