A few years back, I looked into the buyback restrictions because of BS rumors at EESO. At that time, not only could it not be outside of normal hours, it could not happen in the first nor last hour of normal trading. If I recall correctly, it was to assure that the bid/ask values reflected true market sentiment. Since at open some MMs seem to open slightly delayed, and some companies like to drop news at open or close.
I don't have the link anymore, but I'm pretty certain it was on the SEC site. That was my preferred source for such matters.
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