Extremely Rare?

I just found this Silver Rupee coin of India issued by the East India Company in 1835 having the portrait bust of William IV, but the bust having a mysterious countermark! The coin was struck in Calcutta. Is this specific countermark making this coin very rare? Please let me know if you guys find any clue. Thanking you all for going through this specimen :)

Anybody can make a counter mark in any coin, so please consider this as PMD….

Not only extremely rare but unique. There's probably no other coin that has that countermark in exactly that place.

But that doesn't mean the coin is valuable. Unless it is a countermark added by a government on a set of coins for a numismatic purpose the countermark is just damage (as Sjoelund said) and reduces the value.

bjherbison

 the countermark is just damage … and reduces the value.

Depends on the countermark, the coin, and the collector.

Some countermarks have a wide audience and always bring a decent premium. Consider the JOP Canada dollars.

Some collectors enjoy countermarks in general, and some countermarks are widely attributed in catalogs. They can be a form of Americana, or political messages, or advertising, etc etc with some appeal to some collectors. 

I dont recognize the mark on OP's coin, but agree that they are very easy to add at any time by anyone so unless there is provenance I doubt this mark adds any value. Whether it detracts from the value is up to the buyer / collector.

I see, very informative discussions.

Perhaps you should change the title to something more meaningful such as: Unknown countermark.

I have an old thread where I discuss countermarks of all sorts, but most links are broken. I have started to fix only a few of them so far. Still, the basics are all there. You'll find many possible reasons why someone would want to stamp coins or other metallic objects with such a countermark.

Your countermark was done in recess on a punch with a rectangular surface. It seems to be a reasonably good quality punch, which means that the design was meaningful to at least one person. 

However, I can't make sense of that design. Maybe it was meaningful to a silversmith, jeweller or clockmaker and his patrons. Once the coin left the area where it was stamped, the countermark may have lost its meaning. It's exactly what happens with many advertizing countermarks that were meaningful only locally.

Currently, without knowing the meaning of this countermark, and given that this a lower grade coin, it's worth little more than its weight in silver. If the coin had been high grade, the countermark would have reduced the value of the coin.

If, however, the countermark could be identified —and there's one in a thousand chances it will ever be— then it gains more value. I see that you are located in India. Since this is an Indian coin, it's probable that the countermark is Indian. If it was my coin, I would try to find listings of silversmiths, jewellers and clockmakers in India. I would also look quickly at catalogues of 19th-century Indian silver objects such as cutlery or teapots.

EDIT — If you really want to go further, you can also look at the search results for Indian countermarks in the Numista library.

You've explained it well, nice to go through your words. Thank you so so much for these links mate :)