Assessments in proportion to premium notes
319Any assessment shall always be in proportion to the amount of the premium notes held by the insurer having regard to the branch or department to which the policies respectively appertain, but where an insurer alters its rate and still holds in respect of subsisting contracts premium notes at the prior rate, the insurer, as between the respective premium notes so differing in rate, may make and levy such differential assessments as will, in risks of the same amount and of the same class of hazard, equalize the cost of insurance to the makers of the respective premium notes.