It's from Great Britain...
The coin reverse in question is a British half crown. It is KM#835. A half crown measures 32.3 mm.
It weighs 14.138 g. and is .500 silver. The obverse is George V, facing left. The reverse is a quartered shield flanked by monograms.
The obverse designer: Bertram MacKennal.
It was common coinage for Great Britain, and had a mintage of 18,763,000. It has a book value of $5-$35 US dollars.
It has gone through a few changes from ruler to ruler. The next was a George VI. It was KM# 856, 866, 879, and 893.
Here is a 1961(QEII) half crown reverse as it was until the end. This is KM# 907.
I hope this is some help?
Good Luck in the competition!