Each employee of the Superior Court shall be credited for retirement purposes for his entire period of full-time service as an employee of a municipal, city, police, justice or traffic court, provided he shall pay into the retirement fund the contributions he would have paid if he had been a state employee during the period of such court service and his salary for such service had been paid by the state, with five per cent interest on such contributions from July 1, 1961, to the date of payment.