Flat Transfer Query On Basis Of Divorce Decree.

Husband and wife were joint holders in the flat before divorce and the husband had transferred this flat share to his wife by providing an application to the society, couple of years back. This transfer was done internally in the society. Also, as per Divorce Decree he had agreed that he has no rights on this flat including residential right. However, there was no deed signed after divorce, as we were unaware of the procedure. How can a wife get this flat registered on her name as there is no Deed signed between husband and wife after the divorce. She only has Divorce Decree which mentions that ex husband had released all his rights. Also, in case if we don't have any contact details i.e. neither ex husband's contact no. and not even his address, as the divorce was done around 15 years back, than in that case how can we get this flat transferred and registered on wife's name.