Section 12C. In any appeal relative to the assessed valuation of property, the reports of the ratios which assessments in the city or town bear to the fair cash value of each class of property therein and the ratio which the total assessed value bears to the total fair cash value therein as determined by the commissioner's report made pursuant to sections 10 and 10C of chapter 58 shall be admitted into evidence by the appellate tax board on its own motion and shall be prima facie evidence of the assessment practices of the city or town and the ratios at which property is assessed for the year for which said determination is reported by the commissioner and for each following year until a new determination is reported by said commissioner under said section 10C.
The board may give an abatement, provided the evidence so warrants, on the basis of a disproportionate assessment even though that issue has not been raised by the pleadings.