Transfer of property to wife after husband’s death.

1BHK purchased in 1988 in resale for 3.00L in Mumbai suburbs in the name of wife(6th owner of the flat). Only 1 original sale of agreement avail in the whole chain of agreements. Stamp duty paid for 1100/- with receipt available but lawyer says it is under stamped. Proper was transferred from wife to husband via transfer deed in 1989 with consideration mentioned the deed for 90000/- as loan was applied by husband from his company for which property was required in his name. Loan has been repaid and all necessary documents avail wrt closure of loan. Encumbrance certificate also avail. Husband passed away in 2021. Will not done but nomination done to wife,daughter and son(all legal heirs) in percentage of 40%,30% and 30% respectively. Husband’s mother passed away before 1988. Daughter and son willing to give NOC to relinquish their rights without any dispute or consideration. Is my flat under stamped? If yes, should i pay stamp duty as per applicable rates or on pro rata basis as per yearly rates from 1988 to 2021? Is stamp duty applicable on transfer of property done in 1989 from husband to wife? Kindly elaborate the procedure to transfer of the property from husband to wife after death of husband.