Unregistered release deed is in admissible in evidence 

it is admissible only for collateral purposes to show defendant possession of property 

1.  Unregistered Release Deed can't be adduced as evidence.

2.   If the defendant holds registered gift settlement and partition deeds based on the unregistered Release Deed, then the case may not succeed.  However, unless the corresponding documents are studied, it would be not possible to give the correct opinion.

Even though the unregistered release deed was made around 40 years ago, nobody challenged it for the last four decades and the subsequent registered transfer deeds though was in the knowledge of them, nobody objected nor challenged them till date.

Hence the suit for partition by the so called shareholders at this stage may not be maintainable.

Dear client, an unregistered deed cannot be admissible as an evidence but can be used only for collateral purposes.

It’s only admissibilie if impounded and paid the stamp duty and registration 

The bar of limitations starts from knowledge of the same

Un registered release deed is in admissible in evidence 

once there is registered partition deed court would dismiss case filed for partition 

i am not able to understand what you are asking

but for your purpose, s.49 of the Registration Act is relevant which is reproduced below:

REGISTRATION ACT, 1908 Section 49 - Effect of non-registration of documents required to be registered

No document required by section 17 1[or by any provision of the Transfer of Property Act, 1882 (4 of 1882)], to be registered shall--

(a) affect any immovable property comprised therein, or

(b) confer any power to adopt, or

(c) be received as evidence of any transaction affecting such property or conferring such power,

unless it has been registered:

1[Provided that an unregistered document affecting immovable property and required by this Act or the Transfer of Property Act, 1882 (4 of 1882), to be registered may be received as evidence of a contract in a suit for specific performance under Chapter II of the Specific Relief Act, 1877 (3 of 1877)2, 3[***] or as evidence of any collateral transaction not required to be effected by registered instrument.]

also if the release deed is unstamped then under the stamp act too such document cannot be admitted in evidence unless the duty with penalty is paid. 

as regards your query on bar of limitation, only God can explain what you seek to ask by that

- If the defendant is in continuous possession of the property since last 40 years , and the plaintiff was silent within this long period , then the case may be towards the defendants due to being time barred of 12 years period. 

- However, the adverse possession ground is not applied in the case of family property 

- Further, an unregistered released deed is not admissible before the court , but it can be produced for proving the case. 

The question is not about the 12 years limitation period.

It may be seen that the  possession is for more than  40 years and also there were registered transaction pertaining to this property subsequently without any hassle hence the court  may not  hold this possession as illegal.

Therefore there chances that the  plaintiff may not  be able to get a favorable decision in his favor  

The statute of limitations for a suit for possession of an immovable property or any interest therein based on title is 12 years. Article 64 of the Limitation Act of 1963 governs the period of limitation for suits concerning the possession of immovable property based on previous possession and not on title. The plaintiff must file the suit within 12 years from the date of dispossession.