Edward I Farthing of the rare London mint, Londoniensis with no inner circle. Spink 1445A
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All the best,
Clive.

What is the diametre of this piece in MM? I have never owned anything less than a penny, I wanted a farthing, but they are all scarce to find given their diminutive size.

It is roughly 10 mm in size, some of my rarer Anglo-Saxon fractions, i.e. Harold I, Offa etc... are a lot smaller!
Edward's farthings are scarce to find and especially this type as it replaced Civi Tas Lon Don with Lon Don Ien Sis. This type frequently has what I call a 'scruffy portrait'.

That is a lovely example

Cheers!