I found this coin in a coin lot I purchased recently - It is the size of a penny coin and as I know there was no pennies struck in that year for George III, I had difficulty identifying it.
The coin measures 32mm across and weighs 11.9g (my scale might be slightly off).
Agree with the last poster. It was probably a silver plated copy someone was trying to pass off as a half crown. I had a shilling coin that was the same a couple of years ago