Promotional examinations-Credits to veterans.) The board of fire and police commissioners shall give preference for promotional appointment to persons designated in Section 10-2.1-10 whose names appear on promotional eligibility registers by adding to the final grade average which they will receive as a result of any promotional examination 7/10 of one point for each 6 months or fraction thereof of military or naval service not exceeding 30 months. The numerical result thus attained shall be applied by the board of fire and police commissioners in determining the position of such persons on any eligibility list as the result of any promotional examination held for purposes of preference in certification and appointment from such eligibility list.
     No person shall receive the preference for a promotional appointment granted by this Division 2.1 after he has received one promotion from an eligibility list on which he was allowed such preference.