Registration of flat purchased in 1980

Sir

my father in law had purchased a property in mumbai, maharashtra in 1980 on stamp paper of rs 5/-

the society got registered in 2002

now society is saying to do regisration

i approached to sub registrar they said registration is not possible as both buyer (my father in law) and seller (builder) have to be present, 

i am confused, was registration compulsory at that time?

if no, please provide any circular/rule so that i can show to society

please help

regards