Hello Rob
Yes, it is slightly unusual for a sweetheart but not out of the question I suppose. It does look to be a genuine, period conversion unlike some of the military sweetheart buttons that have been appearing on ebay recently which were probably knocked up in somebody's shed last week!
Interesting to see that the backmark is for "Ranken & Co Calcutta", so possibly supplied originally to a diplomat in the Indian Civil Service.
Roger
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