Someone has took a genuine 1820 coin and altered it to 1817 then cast it i assume.
Anyway....I found the paper's that Gary Oddie wrote if anyone is interested. They are all on the link here...https://britnumsoc.blog/2021/10/09/counterfeit-shillings-of-george-iii-1816-1820-iv-a-contemporary-mould-revisited-gary-oddie/
Yes, Gary Oddie passed away quite recently.
@seuk is Peter Poulsen. Most of his website text has been archived, but unfortunately none of the images:
web.archive.org/web/20150825194438/http://www.steppeulvene.com/index.george_iii.html
I hope he's alright.
Sad news about Gary. I never met him but heard lots of good things about him. He was born in the town i reside in.
I just dug my shilling out as this post had sparked my interest. Never even noticed it before but it's got a clear overdate. 1817 over 1820. That's something i have never seen in a counterfeit coin before.