Each county officer may and all other county employes shall be required to become a member of the retirement system established by this act at the date specified in the resolution establishing the retirement system, and thereafter when first becoming a county employe. Those becoming members who have been at any time county employes prior to the first Monday of January of the year the retirement system is established shall be known as original members; and, those becoming members after that date as new members. Any person who becomes a county employe subsequent to the time the system was established, and who has been an employe of the county at any time theretofore, shall receive credit for the service prior to the time the system was established, and shall be known as an original member if he shall contribute to the fund the amount which would have been contributed if the retirement system had been in effect during such previous employment; in which event, the county shall contribute for each employe the amount which it would have contributed if the retirement system had been in effect during such previous employment.