The average specific gravity of the distributed gas tested at each of its testing stations required herein by subdivisions (a) through (d) of section 229.50, as determined by the tests required herein by section 229.21, shall be computed and recorded for each day and each calendar month by each company. In each instance of a testing station where a recording gravitometer is maintained and operated, a continuous record of the specific gravity of the distributed gas, as determined by said recording gravitometer, shall be maintained and the corrected average specific gravity of the distributed gas be computed and recorded for each day and each calendar month from said continuous record except that for each day that the corrected maximum specific gravity exceeds the corrected minimum specific gravity by two or more per cent of the latter, the corrected average, maximum and minimum specific gravity of the distributed gas be computed and recorded from said continuous record. A copy of each specific gravity record shall be maintained at its originating testing station.