The 45 day ownership rule relates generally to imputation...

The 45 day ownership rule relates generally to imputation credits received with dividends, not just to special buy back dividends. It requires that to qualify for the imputation credit you must have owned the stock for 45 days not counting the date of acquisition or disposal. This can be any 45 day period starting on, ending on or spanning the dividend date.

In the case of a share buyback of course your ownership would end at the buyback/dividend date so you would need to buy the stock at least 45 days before the buyback date to qualify.