Tony the only thing i can find is calcutta mint has no mint mark.Bombay mint is a dot bottem on back. And there was a restrike. It should be copro nickel. Could it be a test , for the dies,or start up in pressing?
I have seen several over the years of hunting for good quality for a good price. I've noticed a few being sold as mint trials in zinc. Not sure like I am always cautious with coins of an incorrect compound if its not been listed. I something like this would be recorded. Also other trials of that era tend to have trial piece struck on don't they ?
