As used in this chapter, unless the context otherwise requires:
(1) “Level or stipulated rate assessment” insurers are those incorporated mutual insurers granting insurance benefits on the assessment plan and which collected from their membership a level or stipulated monthly, quarterly, semiannual, or annual assessment or premium, which assessment or premium is not made contingent upon the happening of a certain event, but is based upon stated periodical rates and charges estimated to be sufficient for the payment of all claims and expenses; and
(2) “Pro rata assessment” insurers are incorporated mutual insurers which operate on the plan of calling assessments to pay benefits promised when the contingency insured against arises and which place their membership in groups or circles for the purpose of assessment and collection of dues.