300 examples struck for inclusion in the British Coronation sets of 1839 and 100 other were sold up to 1841. It is speculated, but not confirmed, there may have been some single (not sets) coins sold too, however no concrete info is available if this actually happened.
To answer your specific question, they were a coronation issue marking the 1937 coronation.
From a now disappeared article (I had saved a copy):
Here is the interesting - little known history behind the coin - Pistrucci actually designed the first version of the coronation coin - but people hated it: