Legal heir certificate

It is called "Succession Certificate".  It is given by a court of law. You may approach a lawyer, who specialises in such matters. He will guide you.

The extent rules/forms should be available in O/o MHADA.

These might also be available on website.

GO thru the same.

The rules usually describe requirement in forms e.g; In case of Nomination or NO nomination etc .............

In your case if Succession Certificate/Heirship certificate is mentioned then as suggested by Dr. Ramani take help of LOCAL counsel specializing in such/civil matters.

Mill worker category allotment of MAHADA flat if this way it is done , sorry myself not studied that agreement and details properly but I have have heard. 
 

Generally MAHADA have its own sub rule and regulation as per MAHDA act the way it can be easily transferred to person who are residing and also legal heirs.   

So once you need to visit MAHADA office to check how to transfer ( For flat alloted to mill worker category)

This is separate category of flats alloted . 

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There are other simple way too like giving paper publication etc calling for objection and society then transfer the flat with MAHDA consent

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If ther eis further problem then you need to use Bombay Regulation VIII 1827 and get heirship certificate for flat 

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But initally again Adv need to check the papers regarding Mill worker allotment of flat . (I read it is part of larger settlement between Mill worker , Industralist  , land owner (Govt) , other labour dispute etc. ) 

Means flat are alloted but since no one had money to purchase flat etc , so they came out that specific area of mill or some land will be alloted to mill worker , some one industralist or contractor will construct building and give flat , all labour dues to settled at same etc. 

So without reading the matter jumping is not wise