G'day, PISTRUCCI is famous for his portrayal of St George, and an effigy or two, but I've never before heard of him associated with Britannia.
Wikipedia says this -
"... Although the archetypical image of Britannia seated on a shield first appeared on Roman bronze coins of the first century A.D. struck under Hadrian, Britannia's first appearance on British coinage was on the farthing in 1672, though earlier pattern versions had appeared in 1665, followed by the halfpenny later the same year; the model used, then and later, was Frances Teresa Stuart, the future Duchess of Richmond. Stuart was famous at the time for refusing to become the mistress of Charles II, despite the King's strong infatuation with her. She then appeared on the British halfpenny coin throughout the rest of the 17th century and thereafter until 1936. When the Bank of England was granted a charter in 1694, the directors decided within days that the device for their official seal should represent 'Brittannia sitting on looking on a Bank of Mony' (sic). Britannia also appeared on the penny coin between 1797 and 1970, occasional issues such as the fourpence under William IV between 1836 and 1837, and on the 50 pence coin between 1969 and 2008. ..."
http://en.wikipedia.org/wiki/BritanniaOn that basis, I doubt that PISTRUCCI is much help in resolving this question.
Also, the British coinage shrank over the years:
the copper coins of 1797~1799 were based on the formula: One penny weighs one ounce.
So, the tuppence, at 57 grams, was one of the biggest coins ever issued.
The coins of 1806~1807 were smaller; and
by Queen Victoria's reign, dimensions had settled where they would remain for a century.
The British shifted from copper to bronze about 1860: I can't recall the precise year.
Regarding your 1916 farthing: in WW1, British soldiers fought, and died, for sixpence a day, so your farthing was 1/24th of a day's wages when it was minted.
Peter in Oz