Reference for Menachot 103:18

מאי הוי עלה אמר ר"נ בר יצחק ת"ש דתניא שלימה שחרית ושלימה בין הערבים

Now if it were so, that it was dispensed with in the evening, then it would sometimes happen that the eighth did not bear the [High Priest's] meal-offering, for example, at the time when the High Priest died and they did not appoint another in his stead.

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