1. in this condition society cannot demand to produce possession letter of previous owner.
2.If the previous owner had fulfill all the requirements and got all the amenities in his name then it shall be presumed under section 114 of the evidence act that he received possession letter from the builder. Because without having the possession letter the previous owner could not get all these facilities.
3. you said that you have already getting monthly maintenance bill from the society in your name so it is group from the conduct of the society that you have taken possession about the property in lieu of possession letter of previous owner.
4. conduct of society is contradictory because in one hand it demands letter of position from previous owner and on the other hand issued maintenance bill in your name.
5.if society is registered under the society registration act then you should approach the High Court for issuing of writ of mandamus against the society to stop harassment and issue shares certificate.