rectification without seller and his legalheirs

Actually there is no way except filing a declaration suit in court.  But some State Governments studied like these issues, instructions have been issued to file a notarized declaration that the whereabouts of the sellers/legal heirs are not known, the buyer can make a rectification deed with a proof for the rectification.

What is the error / mistake requiring rectification and does it affect title of the property stated to have been sold / purchased 50 years ago ?

Presumably there should not be any glaring mistake / error in the document.

However, it is better to show the title document to a local prudent lawyer for proper analyses and advise to proceed or ignore.

A decree is passed declaring document to "so and so"  (as pryaed to be rectified) is made by civil court on the subject matter which is binding and final on the concerned parties including Revenue Authorities (responsible to register a document produced before concerned Registrar / Sub-Registrar). 

If you are the interested party and wants to get the error rectified then you may have somehow to bring some of the legal heirs of both the sides to get the error rectified. 

You can issue a legal notice to the seller at his last known address, let it be returned undelievered, then you approach court with a suit for specific relief and seek  court to intervene and execute  a registered rectification deed rectifying the error so found by attaching all relevant documents to prove the error  after which you can get the relief of rectifying the error.

will court rectify it? In absence of legal heir

The court will pass orders based on the merits of the suit filed by you, it may or may not be in your favor.

my question is will court rectify the mistake since legal heir is not available based on merit and proof subnitted.

You first file a suit under specific relief act seeking the relief and convince the court about your genuineness, let the court pass a judgement, you will come to know further course of action after that.

Don't expect the answers for all questions now itself, you first take steps after which you can seek assistance or guidance on further issues if you are stuck in between.

Prima facie  facts posted are based on assumptions and presumptions, isn't it?

However, it is better to consult a local prudent lawyer with relevant records, if there is some truth in the story.