In computing the length of service of a contributor for retirement purposes, full credit shall be given to each original member for each year of service rendered to the county prior to January first of the year the retirement system was established. This shall include the services of a county official whose compensation was in the form of fees collected by his office, and shall also include the services of employes paid directly by such county official out of such fees. Full credit shall also be given for each year of service of a contributor who was a per diem employe of the county for a period of at least five years prior to the time the system became effective and who averaged at least two hundred days of employment in each of such years. As soon as practicable, the board shall issue to each original member a certificate certifying the aggregate length of his service prior to January first of the year the retirement system was established. Such certificate shall be final and conclusive as to his prior service, unless thereafter modified by the board upon application of the member. Any member who had been employed on a part-time basis and had been excluded from membership in the plan and who completed more than 1000 hours of services during the 12-month period beginning on the first day of employment or in any succeeding 12-month period thereafter shall receive service credit for a fractional portion of a year determined by the ratio of the number of hours of service credited in that 12-month period to 1400 hours for the purpose of the computation of his retirement credit upon payment by the employe of the amount which the board determines should have been the applicable member contribution during such period. Such payment may be made in installments over a period fixed by the board. The time during which a member is absent from service without pay may be counted in computing the service of a contributor, if allowed by the county commissioners and approved by the board and if the employe pays the member contribution, including the pickup contribution, and the county's contribution into the fund based upon the contributions made the year immediately preceding the leave of absence.