Registration / Stamp duty required for transfer of shares in Coop Hsg Society?

If A who is a member of a Coop housing society transfers his shares to B who is not a member, then B must become a member of the society and pay Society transfer fees of Rs 25000 in Mumbai. But the shares convey the interest in the Society and occupation rights in the flat, not the title so technically B is not buying the flat. What kind of agreement / deed is required and is stamp duty, registration fees payable - if so computed on nominal value of shares or market value of property? Any case law or relevant law on the subject? Thanks