There are three variant mintings of the 1918 and 1919 British Penny. The pennies minted in London have no mintmark.
The mintmarks "H" for the Heaton Mint, Birmingham, or the "KN" for King's Norton Metal Company are to the left of the date on the reverse and are extremely small.
King George V
Quite aside from the fact that all 1911 British and British Empire Pennies feature George V and have GEORGIVS V inscribed on the coin, if the head is facing towards the left then it is George V. If the head is facing to the right it is Edward VII.
1936 was the last year British coins were minted with the likeness of George V. The 1936 Penny is far from rare and you should have no trouble finding one.
Pre-decimal British Pennies were made from bronze from 1860 until 1967.
If the coin has no country name on it, it is British, otherwise, it could have come from any of the 50 plus British Empire/Commonwealth countries.
A British 1912 bronze Penny (George V)(minted at Heaton - mintmark = H), uncirculated and in absolute mint condition could fetch up to £250 GBP. If it has been circulated but still in good condition, it might fetch anything from £10 to £90 GBP. A British 1919 bronze Penny (George V)(minted at Heaton - mintmark = H), uncirculated and in absolute mint condition could fetch up to £800 GBP. If it has been circulated but still in good condition, it might fetch anything from £15 to £250 GBP. The values quoted are the best possible for the best specimens in their grades, but may be significantly lower due to varying demand for the coin and the prevailing economic climate. A reputable coin dealer will be able to give a more accurate valuation based on inspection of the coin.
I am unfamiliar with the reference to "centre diamond" on an Australian George V Penny. Please provide more information.
You are thinking of coins of George V. Only one nation has not demonetized George V coins: Canada. This is the only place in the world that a George V coin can legally circulate. However, the George V farthing is almost the same size as the British penny and can VERY RARELY be seen in circulation.
King George V
Yes. George V died in January 1936 but the coins had already been minted. The British West Africa Pennies that were issued and circulated had both George V on the earlier coins, and Edward VIII on the coin issued later.
It depends on the nation.
In 1919 the British Monarch was George V.