Formerly, I was a member of a Massachusetts contributory retirement system and withdrew my funds. Now, can I “buy back” that prior service and enter the retirement system at my earlier membership date and lower contribution rate?

While you may “buy back” that time in order to add to your creditable service, your “buy-back” does not entitle you to your earlier membership date or contribution rate. In other words, you may purchase your service time only; however, not the benefit of the earlier membership date and lower contribution rate.