Section 21A. In any city or town which accepts this section, an assessor or assistant assessor who has completed the necessary courses of study and training and has been awarded a certificate by the International Association of Assessing Officers as a certified assessment evaluator or who has been awarded a certificate by the Association of Massachusetts Assessors as a certified Massachusetts assessor shall receive as compensation from such city or town, in addition to the regular compensation paid by such city or town for services in such office, an amount equal to ten per cent of such regular compensation; provided, however, that in no event shall such additional compensation exceed one thousand dollars annually, if such assessor or assistant assessor is employed on a full-time basis, or five hundred dollars, if such assessor or assistant assessor is employed on a part-time basis. An assessor who has been awarded both certificates referred to above shall receive such additional compensation for only one of such certificates. In order to qualify for such additional compensation, an assessor or assistant assessor shall submit proof that he has been awarded either or both of the aforesaid certificates to the mayor or the board of selectmen of such city or town. The additional compensation herein provided shall be prorated for any twelve month period in which an eligible person does not hold the office of assessor or assistant assessor for twelve consecutive months.