There were no British "v" pence coins minted in 1820, or ever.
Hamengkubuwono V was born in 1820.
Such a coins does not exist. All 1901 British coins featured Queen Victoria on the obverse. George V started appearing on British coins in 1911.
Robert V. Richardson was born on 1820-11-04.
Patriarch Dionysius V of Constantinople was born in 1820.
The British 1925 gold Sovereign (KG V) is not rare and in circulated condition will probably get only bullion value.
The following British 50% silver Threepences currently have the highest value from the reign of George V, assuming that they are in mint uncirculated condition. 1922 1925 1926 1930 Circulated coins from the same period will have much the same value for each year, something between £1 to £8 GBP if they are in good condition.
If the true value is t and the calculated or measured value is v then absolute error = |v - t|, the absolute value of (v - t).If v >= t then the absolute value is v - tif v
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1760-2010 George III 1760-1820 George IV 1820-1830 William IV 1830-1837 Victoria 1837-1901 Edward VII 1901-1911 George V 1911-1936 Edward VIII 1936 George VI 1936-1952 Elizabeth II 1952-present
A coin marked with "GEORGIUS" or "GEORGIVS" could be any British, British Colonial or British Empire/Commonwealth coin issued during the respective reigns of any of the six King Georges. George (I)(1714-1727) George II (1727-1760) George III (1760-1820) George IV (1820-1830) George V (1910-1936) George VI (1936-1952)
It is expressing a change in the value of something as a percentage of its original value. So, if the original value (V) changes to the new value (W) then the change is W - V. Then, the relative change is (W - V)/V and the percentage change is 100*(W - V)/V = 100*(W/V - 1)
Modified coins have no value as a collectible. Any value would be associated with the necklace.