The excess of the number so fixed for any grade in any category over the number of officers of the Regular Corps on active duty in such grade in such category (including in the case of the director grade, officers holding such grade in accordance with section
207
(c) of this title) shall for the purpose of promotions constitute vacancies in such grade in such category. For purposes of this subsection, an officer who has been temporarily promoted or who is temporarily holding the grade of director in accordance with section
207
(c) of this title shall be deemed to hold the grade to which so promoted or which he is temporarily holding; but while he holds such promotion or grade, and while any officer is temporarily assigned to a position pursuant to section
206
(c) of this title, the number fixed under subsection (c) of this section for the grade of his permanent rank shall be reduced by one.